Dahir Adan was a 20-year-old Somali immigrant. (Gillian Brockell/The Washington Post)

Dahir Adan, a former Apollo High School honor student, walked into a mall here last Saturday and stabbed 10 people before an off-duty police officer shot him dead.

The Islamic State hailed him as one of its soldiers. The FBI has hesitated to describe Adan’s rampage as an ideologically inspired terrorist attack and Thursday night called on witnesses — ideally with videos — to come forward.

For years, FBI officials and other national security experts have said that apparent lone wolves such as Adan and Ahmad Khan Rahami, charged in last weekend’s New York bombing, have no discernible profile that could be used to head off terrorist acts. But some counterterrorism experts who studied attackers motivated by jihadist ideology say they often exhibit traits similar to those of non-Muslim mass killers who have attacked schools, churches and workplaces.

“They share common behavioral and psychological characteristics,” said John Cohen, a criminal justice professor at Rutgers University and the former counterterrorism coordinator at the U.S. Department of Homeland Security. “They’re the same people.”

Many studies of mass killers show that they had serious personal or psychological issues, regardless of their motives or religious identity. They bounced from job to job, struggled to find friends or romantic partners, or felt bullied by their peers. Some had traumatic home lives. Others struggled with a profound sense of shame.

Awareness of these shared traits could help focus FBI questioning of certain young men who already have come to the attention of law enforcement, these experts say.

“The attackers themselves act and feel like victims,” said Adam Lankford, a criminologist at the University of Alabama. “Various words are used. Persecution. Discrimination. Bullying. Humiliation. Mistreatment. The sense that someone else is picking on me and is out to get me.”

Omar Mateen killed 49 people inside the Pulse nightclub in Orlando in June. (MySpace via AP)

Lankford points to Omar Mateen, who pledged allegiance to the leader of the Islamic State during the rampage in which he shot and killed 49 people at the Pulse nightclub in Orlando. The FBI investigated Mateen prior to the attack, determined he had no links to terrorism and dropped the investigation. But Lankford said there were other red flags, such as boastfulness and a craving for attention, which are often seen in mass shooters.

“Omar Mateen has more in common with Dylann Roof than he does with Osama bin Laden,” Lankford said, referring to the white supremacist who allegedly shot and killed nine black churchgoers last year in Charleston, S.C.

Law enforcement authorities face a daunting challenge as they try to identify would-be mass killers before they act. There is a growing awareness among counterterrorism officials that law enforcement can’t do the job alone — that the traditional model of investigate, arrest and prosecute isn’t enough.

Authorities want greater community involvement. That means bringing in mental-health professionals, faith leaders, educators and others. The Department of Homeland Security is administering $10 million in grants to help communities address the threat of violent extremism — though such grants have riled Muslim community leaders who feel like the effort is unfairly directed at Muslims.

“Well-informed families and communities are our best defense against violent extremist ideologies here at home,” said George Selim, director of the Task Force on Countering Violent Extremism, an interagency body created in January to coordinate efforts across government agencies.

The landscape has changed since Sept. 11, 2001: The violent jihadist attacking inside the United States today is less likely to be someone trained overseas and more likely to be a disturbed American seizing on an ideology amid a host of personal issues or psychological problems.

According to the New America Foundation, jihadist-linked attacks in the United States since 9/11 have killed 94 people — and more than half of those were slain by Mateen. Such incidents, though more frequent in recent years, still account for a tiny fraction of the more than 200,000 homicides in the United States since 2001. Meanwhile, this year alone, non-jihadist incidents of mass shootings have killed nearly 100 people.

America’s suspected ISIS supporters

The FBI has between 7,000 and 10,000 open counterterrorism investigations a year. On top of that, it undertakes each year roughly 10,000 counterterrorism “assessments,” a pre-investigative look at a person who has surfaced through a tip as someone who might be up to no good.

On several occasions the FBI has launched at least a preliminary assessment of a potential terrorist and then backed off — only to see that person erupt, months or years later, in violence.

That has included Rahami, whose father told the New York Times that he conveyed to the FBI his concerns that his son was becoming radicalized. The FBI, for its part, said that at no time did the father relay such fears. They questioned Rahami when he returned from a year in Pakistan and knew about his stay in Quetta, a stronghold of the Taliban and al-Qaeda.

The bureau questioned Mateen twice after several people in Florida reported his possible terrorist inclinations. And the FBI questioned Tamerlan Tsarnaev years before the Boston Marathon bombing because the Russian government suspected he had links to Chechen extremists.

Ahmad Khan Rahami has been charged in connection with last weekend’s New York bombing. (Union County Prosecutor’s Office via Associated Press)

The bureau uses behavioral analysis in some investigations in an effort to discern clues to a person’s motivation and intent. But given the volume of probes, it can’t do it in all cases, officials said. And it is sensitive to allegations of profiling and of intruding on privacy.

“We’re not the thought police,” said a senior law enforcement official who spoke on the condition of anonymity because of the sensitive nature of the topic. “So while we aim to be predictive in identifying the threats and how they are going to manifest themselves, we have to be very careful we’re not profiling people based upon their thoughts or utterances — upon First Amendment-protected activity.”

In general, tips about potentially dangerous individuals bubble up through the 56 field offices. Each has at least one Joint Terrorism Task Force (JTTF), which includes state and local law enforcement agencies and personnel who work both domestic and foreign terrorist threats. So a homegrown violent extremist, whether inspired by the Islamic State or by other ideological propaganda, would be on the JTTF radar screen, officials said.

“We are looking for needles in a nationwide haystack,” FBI Director James B. Comey told Congress in July, “but even more challenging, we are also called upon to figure out which pieces of hay might someday become needles.”

The bureau says it has had some success: That very month, Comey noted, the bureau made four arrests of individuals who were “moving on that path . . . to acting on violence.”

The Post looked at 15 attacks in the United States since 2001 in which investigators believed the violence was probably linked to radical Islamist ideology. The cases show no obvious pattern of religiosity, mosque attendance or even contact with militant groups. There is no consistent pattern in age, location or ethnic heritage.

“There’s no usual suspect,” said Seamus Hughes, the deputy director of George Washington University’s Program on Extremism.

Many of the cases involved people who showed signs of psychological or behavioral problems. For example: Nidal Hasan, the U.S. Army psychiatrist who killed 13 people in a 2009 shooting rampage at Fort Hood in Texas, was described by co-workers as aloof, paranoid and isolated.

Naveed Haq, a Muslim man who converted to Christianity not long before he shot six women at the Seattle Jewish Federation in 2006, had been diagnosed as bipolar and long been angered by women who were not romantically interested in him.

Alton Nolen, who in 2014 beheaded a co-worker in Oklahoma, appeared to have been obsessed with beheadings, investigators said.

Wasil Farooqui, who last month stabbed two people in Virginia, told police that he was hearing voices.

Mohammad Abdulazeez, who killed five people in a shooting rampage at military recruitment centers in Chattanooga, Tenn., suffered from severe depression, suicidal thoughts and drug abuse.

And Rahami, the accused bomber, had once stabbed his own brother.

Shame and injustice: ‘The sense that someone is picking on me’

Religiosity is often presumed to be the animating theme in cases of Muslims committing mass violence, but it usually isn’t, experts say.

The Muslim men who have carried out violent attacks in the United States since 9/11 have ranged from deeply religious to nominally so, or what Hughes calls “converts to ISIS, not Islam.” Some have prayed regularly and performed the hajj pilgrimage. Others abused drugs and alcohol.

Omar Mateen professed his allegiance to the Islamic State, but the women Mateen had married didn’t wear headscarves, as the Islamic State would mandate, and former colleagues said he seemed more interested in going to the gym than praying.

The FBI and other agencies have a challenge in separating all this transient rage, alienation and eccentricity from an imminent threat to the public, and particularly in the age of social media and encrypted text messages. The shooters, bombers and stabbers fit a profile more obviously in hindsight.

Clockwise from top left: Wasil Farooqui, Mohammad Youssduf Abdulazeez, Naveed Haq, Nidal Hasan and Alton Nolen. (AP/AGENCE FRANCE-PRESSE VIA GETTY IMAGES)

At this point, very little is known about the most recent one, Dahir Adan, who was 20. In St. Cloud, his quiet town of 65,800, his family kept his burial on Friday a secret. And his close friends, who did not want to be identified, will say only that he was kind; that he was large and muscular but never got into fights; that he tutored other kids and handed out money to those who needed it. They say he loved basketball and his Xbox, and that the Los Angeles Lakers were his favorite team.

“That’s all he ever did was get good grades and play Xbox,” said one friend. “He wouldn’t hurt a fly.”

What they won’t say is whether Adan, who quit his most recent known job in June and failed to enroll in the fall semester of college, was depressed. They won’t say if he was religious; if he was angry about U.S. foreign policy; or if he talked to Islamic State members online or admired their ideology. No one will say if Adan, a Somali-American, was ever affected by the racial tensions and anti-Muslim harassment that sometimes ripple to the surface in this Mississippi River town.

His public Facebook profile contained no jihadist images. He appeared to enjoy soccer. Like many of the other alleged jihadists over the past 15 years, he is now dead. So there will be no confession.

Achenbach reported from Washington and Nakashima from New York. Alice Crites and Julie Tate in Washington contributed to this report.