The police came for him just after 4 a.m.

Wael Ali was still awake, cramming for exams in the small house that served as his sanctuary from the past.

It had been four years since Wael’s identical twin brother, Wasel, was found dead in a wooded area near the Mall in Columbia. To escape constant reminders of the crime, Ali had moved 700 miles away, to Marietta, Ga., where he had enrolled in college and was finally rebuilding his life.

On Sept. 15, 2011, the 23-year-old was staring at his laptop in the pre-dawn darkness when the phone rang. Ali didn’t answer. But when he looked up, he says, he sensed movement outside.

At the back of the house, he heard police radios crackling. A fugitive squad was approaching. The officers had a warrant for his arrest.

From the Cobb County jail, Ali called his family back in Maryland.

“I’ve been arrested for first-degree murder,” Ali recalls telling his father, Gihad.

His father was incredulous. “Who?” he asked. “Who did you kill?”

They said I killed Wasel, he replied.

His father, a tough-minded former colonel in the Sudanese army, sobbed as the anguish of Wasel’s death was rekindled and then immediately eclipsed by the suggestion that he had been killed by his mirror image.

The twins had always been inseparable. Growing up, they’d slept in the same bed, gotten sick at the same time, recited the same prayers at the family’s mosque, enlisted in the Army on the same day. Their names were distinguished from each other by a single letter. Their faces, their close-cropped hair, their skinny athletic bodies — even their DNA — were interchangeable.

Their father didn’t believe “for one fraction of a second,” he says, that Wael had killed Wasel.

Police and prosecutors didn’t believe anything else.

* * *

In 2010, almost 13,000 people were slain in the United States, according to crime statistics compiled by the FBI. Thousands were killed by someone they knew: spouses, children, lovers, friends, and, in just 107 cases, by a brother or sister.

If fratricide is unusual, fratricide among twins is almost unheard of. Dominique Bourget, a Canadian forensic psychiatrist and expert on fratricide, can recall studying only two cases among twins in more than 20 years.

The novelty of such a crime is surpassed only by the difficulty of prosecuting it, especially in the “CSI” age, when juries hunger for irrefutable physical evidence. In the case of the Ali twins, their DNA was identical. Evidence tying one brother to the crime scene could have been left there by the other.

Although Wael was questioned by Howard County police almost as soon as his brother’s body was discovered Aug. 27, 2007, investigators didn’t have physical evidence that he had been at the murder scene. And they had no weapon.

Wasel, 19, had been asphyxiated, an autopsy report said. Investigators would later argue in court that there was only one person furious enough to choke the life out of Wasel with his bare hands: his twin brother, Wael.

* * *

Wasel came first, Wael 15 minutes later. They were born March 16, 1988, in Khartoum, the capital of Sudan.

Their mother, Nagwa Hussein, was a social worker. Their father was transitioning out of army life, and in 1991 he moved to Severn, not far from Fort Meade. “I was looking for a better life,” Gihad says. “I started from zero.”

He opened a translation company and later started a limousine operation. Four years after he left, his family joined him, eventually settling in a Columbia townhouse. The twins picked up English in six months and quickly embraced the trappings of a suburban American childhood, playing sports and watching professional wrestling on TV. They’d sneak into their parents’ bedroom to practice their moves, diving onto each other like little Hulk Hogans.

“I think we broke like three beds,” Wael says.

Friends and family members frequently had trouble telling them apart, unless Wasel wore his glasses. Later, people often distinguished one brother from the other by their manner. Wasel, friends and relatives say, was markedly more serious and introspective than Wael. But both brothers knew how to throw a punch.

In the bedroom they shared as teens, the disagreements between them sometimes grew so intense, Wael says, that they would begin trading blows. At least until they heard their parents coming.

“What’s going on?” their mother would demand.

“Nothing,” they would answer. “Nothing at all.”

The twins formed a tight bond with several neighborhood friends, who called themselves “The Crew.” They hung out for hours at a playground and wooded area near the mall — the same spot where Wasel’s body would be found.

The brothers had always idolized their father’s military service, and they developed a preoccupation with police and military work. Wasel was so fixated on detective work, remembers close friend Cassie Colassard, that as a teenager he carried around a notebook in which he’d write down bits of gossip so he could investigate who was spreading rumors.

“They enjoyed the cop life, the mystery-solving,” Colassard says.

But their fascination grew so intense that it landed them in trouble with the very authorities they admired. After graduating from Wilde Lake High School in 2006, the twins joined the Army, intent on becoming military police officers. But when they returned from basic training in 2007, they allegedly stole an Army sedan from Fort Meade and were arrested, accused of trying to pull over a speeding vehicle on Route 29.

The brothers, wearing military batons and pepper spray on their belts, even called police to report the speeding vehicle, according to charging documents. They identified themselves over the phone as military police officers. The twins were given probation before judgment in Howard County District Court after being charged with stealing a government vehicle, impersonating a police officer and possession of a dangerous weapon.

“We just made some wrong decisions,” Wael, now 24, says. “We got mesmerized by the whole being in uniform thing and being in the Army.”

They left the Army and moved back in with their parents. Wasel got a job working at Banana Republic at the mall. Wael was selling cellphones. But they apparently hadn’t lost their fixation with law enforcement.

In August 2007, the twins were charged with dressing up as police officers again, this time at the Clarendon Ballroom in Virginia. They allegedly told security guards and off-duty police officers that they were assisting Arlington County police with security. When police searched their car, they found an unloaded .45-caliber pistol inside that they would eventually say had been stolen. The brothers were issued a court summons on a charge of impersonating police.

Howard prosecutors later alleged that Wael was furious at his brother over the episode, telling him: “I am so done with you. I’m sick of getting in trouble because of you. Wait till we get home.” Once they got back to Columbia, according to Wael’s statements to police, they got into a fistfight.

Then came the day that would tear two brothers apart, leaving one dead and the other the subject of a police investigation. It unfolded just days after the Clarendon episode, on Aug. 22, 2007, at the family’s townhouse on Warm Granite Drive.

The twins and their father were home that Wednesday morning when a Howard SWAT team raided the family townhouse in an unsuccessful effort to find more guns. Wael recounted what happened during an interview with detectives that was played in court: The twins and their father were handcuffed and left side by side on the floor for hours as police officers in masks searched the home, kicking in every closed door.

“It looked like a tornado went through the house,” Wael told detectives.

An already awful day would get worse. That night, Wasel was called into the Banana Republic store and confronted over an alleged shoplifting scheme, according to police reports. He admitted that he’d stolen clothing and implicated his brother, police said. He agreed to pay restitution but was fired.

Their father sent Wael to the mall in the family minivan to pick him up. Wasel never came home.

* * *

Almost five years later, a Howard prosecutor stepped to the microphone and began to tell a jury a gruesome story.

Wasel’s body was found by a hiker down a small, dirt path in the woods the twins played in as boys. The black suit jacket he had worn to the mall was pulled back over his shoulders. His arms were pinned behind him. His body was beginning to decompose. The cartilage in the bottom of his neck had been broken. It had been five days since Wael had been sent to the mall to pick him up.

“We have alleged that Wael Ali killed his twin brother, and he sits here presumed innocent,” prosecutor Doug Nelson said. “We will chip away at the innocence little by little by little,” he promised, “and what’s gonna be left is his obvious guilt.”

Nelson warned the jury that there would be no DNA evidence, no fingerprints.

“We don’t have DNA at the crime scene because with twins, they share DNA,” he said. “There’s no fingerprints on the murder weapon. There’s no murder weapon.” Just the hands of the killer.

The prosecution’s theory of the case, pounded at throughout the trial, was that Wael was furious at his brother over the Clarendon Ballroom incident and that he became more enraged when their father was humiliated during the search of their house. The prosecution also attempted to show that Wael knew he had been implicated in the shoplifting scheme.

Wael went to the mall. He, Wasel and some friends talked for a while at a cellphone kiosk, mall surveillance cameras showed. Then Wael went to the parking lot, and Wasel eventually left through another exit, the mall cameras revealed.

Authorities asserted that Wael’s account of what he did next — either waiting for his brother, looking for him, sitting at a gas station, driving to Colassard’s house — changed throughout the investigation. Prosecutors argued that Wael found Wasel and that they went to the woods, where a confrontation between them grew so heated that Wael killed Wasel with his bare hands.

Wael’s legal team, led by veteran defense lawyer Jason Shapiro and David Zwanetz, attacked the prosecution’s case as unsubstantiated by facts. Wael left the mall without Wasel after waiting for him in the parking lot. He did not, they said, kill his brother.

“There is no motive present for Wael to kill his twin brother, his best friend, and his flesh and blood,” Zwanetz said in his opening statement.

But prosecutors argued that there were 25 minutes that were unaccounted for after Wael left the mall. No cellphone calls. No one interviewed by police had seen him.

By the time Wael was questioned by detectives, his brother had been buried in a white shroud according to Muslim ritual. Jurors watched a videotape of the interview, which took place eight days after Wasel’s body was found.

“Did you kill him?” one detective asks. “Let us help you if you help us. We’ll be helping each other, right?”

Wael sobs loudly. Several times, he denies killing his brother.

“Why do you keep asking me that?” he pleads.

* * *

The case went cold.

Wael spent a couple years in a deep depression. “For a long time, I said that day I died, too,” Wael says. “I stopped caring for a lot of things. I stopped wanting to be happy for a long time.”

In 2009, he moved to the Atlanta area with his mother and sister. They were opening a Mediterranean restaurant, and he was starting college at American Intercontinental University Atlanta. His major: criminology.

“I wanted to know about the criminal mind,” he explains.

That same year, Howard’s cold-case investigator, Nick DeCarlo, was assigned an unsolved crime: the death of Wasel Ali.

DeCarlo reinterviewed Wael’s friends and reviewed cellphone records, trying to pin down what Wael was doing in those unaccounted-for 25 minutes after he left the mall. He thought it was suspicious that after those minutes went by, Wael started repeatedly calling Wasel’s girlfriend and other friends, asking whether anyone knew where his brother was.

In August 2010, Wael agreed to meet with DeCarlo during a visit home to Columbia. The jury listened to a tape of their encounter in a police interrogation room.

“You don’t know what it’s like to lose a twin brother,” Wael tells DeCarlo. “Please do not insult me and come at me as a suspect. I have been through hell and back. . . . There is no reason why I want my brother’s killer to just be out there walking the streets.”

“Neither do I,” DeCarlo replies.

“My life has changed forever,” Wael says. “You don’t know what it’s like to get up every day expecting him to be right there.”

There is a long pause. Then another detective begins reading Wael his Miranda rights.

“You have the right to remain silent,” the detective says.

“Am I under arrest?”

DeCarlo: “You are not under arrest.”

Wael: “Oh my God.”

DeCarlo: “Wael. You are not under arrest. You came here voluntarily, and you are free to go.”

Wael calms down.

“It’s very simple,” DeCarlo continues. “Two brothers. You have a fight. You have some issues. You are going to resolve the issue as brothers often do, with fists or whatever, and unfortunately Wasel got hurt and he died.”

“We did not fight,” Wael insists. “My brother and I did not fight that day.”

DeCarlo asks why he was so concerned about his brother in the hour after he left the mall, making calls so frantically.

“There was a lot going on that day,” he says. “We had a lot to do, a lot to take care of at the house.”

“I know you did,” DeCarlo counters. “You took care of it in the woods.”

“I swear to you: We did not get in a fight.” And then he raises his voice: “I did not kill my brother!”

The interview left the courtroom silent.

During cross-examination by Shapiro, DeCarlo and the prosecution acknowledged that they didn’t have police notes from the 2007 Clarendon Ballroom incident mentioning the alleged threat Wael had made to his brother. The anecdote was relayed by the original investigating officers after DeCarlo contacted them.

Shapiro also pressed DeCarlo to acknowledge that he had no direct evidence that Wael had accompanied Wasel into the woods where the body was found.

“In fact, it could have been that Wasel was in the woods and the killer came upon him in the woods, correct?”

“I think it’s highly unlikely,” DeCarlo answered.

“I understand that might be what you think,” Shapiro said, “but that’s possible, correct?”

DeCarlo’s answer: “Possible.”

* * *

As the jury deliberated, Wael went back to his jail cell and prayed.

Several of the twins’ friends had testified about their relationship and about Wael’s actions the day Wasel disappeared, including Colassard.

“In my heart, I knew there was no way that Wael was his killer,” she says.

His parents remained loyal, too. “Everybody knew,” Wael’s mother says, “how much they love each other.”

Inside the jury room, only a few jurors initially leaned toward a guilty verdict, according to juror Sandra Rivers, a Columbia resident.

“They thought he could have, might have done it, but [prosecutors] didn’t prove it,” Rivers says. “I just couldn’t believe that this person, who seemed to be so close to his brother, had killed him.”

But one juror wouldn’t budge from guilty because of Wael’s phone calls, Rivers says.

After 21 hours, the jurors told Judge Richard S. Bernhardt that they couldn’t reach a verdict. It was 11 not guilty, 1 guilty.

Bernhardt sent Wael back to jail while authorities decided whether to retry him. Howard prosecutors declined to comment about the case or the decision they reached a week later: They would not retry him.

Wael’s sister called him in jail with the news. “I just started screaming,” Wael remembers.

On April 16, 2012, his family, Colassard and his attorneys greeted him in the jail’s parking lot. He had been imprisoned for almost a year. His mother, her hair covered by a head scarf, kissed both of his cheeks, squeezing him tight. His father kissed him, too, and they rocked back and forth in a long embrace. In their minds, he’d been exonerated.

But, in a statement, Howard police said the case remains active: “The investigation to date has continued to lead detectives to the victim’s brother as the main suspect. We will continue to investigate the case, and if additional leads are discovered, we will review with the state’s attorney’s office.”

* * *

The twin boys were racing around the picnic pavilion at Cosca Regional Park, and Wael stopped to watch. His sister’s sons, Ali and Omar, 5, are identical twins. And they are inseparable, each unable to imagine life without the other.

As the Alis gathered in Clinton on a summer Sunday to celebrate Wael’s release from jail, his very presence remained an acute reminder of Wasel’s absence. Or as their mother says: “Whenever we see Wael, it’s always a cause of pain. We suffer more just seeing him.”

At the park, the smell of lamb drifted through the air as their father sat at a picnic table scrolling through old family photos on his iPad.

There was his wife, young and beautiful, her smile filling the screen. There were the twins as little boys.

“This is the one who died,” Gihad says, pointing at Wasel. Here is a picture of Wael “after the incident,” as his father puts it.

He says he thinks the past five years have been a test of faith from God. He believes his family has passed. Still, he worries about Wael, who was making plans to return to college. “This has been a disaster for him,” his father says. “But I think he is getting through this.”

Wael arrived at the picnic wearing dark sunglasses, jeans and a New England Patriots hat.

Relatives, who had flown in from all over the country, embraced him. Members of the local Sudanese community dropped by. His defense attorneys and Cassie Colassard came, too.

If any of them harbored doubts about his role in his brother’s death, they were carefully hidden.

The outside world is a different story, Wael knows. “I left this area because I got tired of the looks I got — the pity, the sympathy,” he says. “Now I have to deal with people second-guessing me — ‘Did you do it?’ ”

But living with suspicion isn’t as hard as living without his brother. He sees Wasel every time he looks in the mirror.