From the time a suspect was arrested and then convicted in the 2017 murder of Christian Matthews, two key questions remained unanswered: How did the killer slip undetected into a Maryland townhouse where the 17-year-old had been sleeping? And how did he slip back out?

Turns out, authorities assert in new court documents, the victim’s twin sisters were in on it.

“This was a planned attack,” Kelly McGann, Montgomery County assistant state’s attorney, said in court recently. “It was callous. It was vicious.”

Matthews died of asphyxia. Authorities say he was strangled in the basement of the Colesville-area townhouse on March 21, 2017.

One of the new defendants, Leaundra Matthews, 23, appeared in court briefly Friday afternoon and was ordered to remain held on no-bond status on charges of conspiracy to commit first-degree murder and first-degree murder. She and her sister, Lemae Matthews, were 18 at the time of their brother’s death.

Detectives have long suspected others were involved. As they worked on the case, the new court filings assert, they learned that Lemae Matthews had been watching a television show called “I (Almost) Got Away With It” in 2019.

“You all dodged a bullet,” a family member of Lemae Matthews told her, according to investigators, “and I need to know you did it. I need to know. I know it has to be weighing on you.”

Lemae Matthews, according to court documents, then allegedly whispered back: “I’m sorry.”

Later, as another relative joined the conversation, Lemae Matthews said she suggested to her sister that their brother could have just been beaten up.

“No, he has to go,” Leaundra Matthews allegedly responded, according to charging documents.

Reached by phone Friday, the twins’ mother declined to comment: “I don’t really know what’s going on now,” she said.

Court records do not indicate if Leaundra Matthews has retained an attorney.

As for her sister, she was arrested on the same charges in June. At the time, an attorney representing Lemae suggested the case against her was shaky.

“The allegations against Ms. Matthews only suggest peripheral involvement,” the attorney, Lucy Larkins, said in court. She added that the statement of probable cause against Lemae Matthews was incomplete and took the word of at least one troubled witness at face value.

“The statement of charges doesn’t really add up,” Larkins said.

On July 23, prosecutors dropped the first-degree murder charge against Lemae Matthews, according to court records. She remains jailed on no-bond status for the remaining conspiracy count, the records show.

The first person arrested in the case in 2017, Tysean Lipford, had been the boyfriend of Leaundra Matthews. His motive centered on concerns that Christian Matthews had reportedly said he wanted to hurt Leaundra Matthews and wanted to hurt the young child of Lipford and Leaundra Matthews.

Lipford was tried in the case, convicted of second-degree murder and sentenced to 30 years in prison.

The new developments add a fourth defendant in the case: Lipford’s close friend in 2017, Daniel Howard, who is now 25. Howard was indicted last week on murder and murder conspiracy counts, according to court records, and remains held on no-bond status.

The new records allege that on the night of the murder, the twins unlocked the backdoor of the townhouse and left disposable gloves for Lipford and Howard. The two men then came in and slipped into the basement, police say, as Christian Matthews was sleeping.

A short time later, detectives say, at least one person in the home heard the teen start screaming.

“Christian Matthews broke away from Daniel Howard and Tysean Lipford and tried to run upstairs,” detectives wrote, citing an account that Lemae Matthews allegedly gave to a family member. “But Tysean Lipford slammed the door shut, preventing his escape.”

Christian could then be hear yelling “Stop! Stop!” before his screams got quieter and quieter, police said.

Attorneys who’ve represented Howard in court also have questioned the strength of the case, noting the detectives interviewed him 2017 but chose not to arrest him.

“This case is four years old. The evidence is four years old. There is nothing new,” Asher Weinberg said in court this summer. “If they thought he was a danger, if they had a real reason to believe he was involved, there was no reason not to charge him back then.”

The case began for police at 2:25 a.m. that March day when they were called by the twins to the family townhouse on Mondrian Terrace. Officers were met outside by Leaundra Matthews, who was standing near her car. She said that shortly before calling 911, she’d been in her upstairs bedroom with her twin sister and her sister’s baby when they heard “a loud commotion” coming from the basement area, according to charging documents.

When police arrived, Lemae was sitting inside Leaundra’s car with her sister’s baby and their two younger sisters.

Officers went into the townhouse basement and found Christian Matthews unresponsive. He was taken to Holy Cross Hospital where he was pronounced dead. An autopsy revealed a fracture to a bone in his neck.

Detectives learned of earlier tension between Christian Matthews and Leaundra’s boyfriend — Lipford — and brought him in for an interview that lasted more than four hours. He ended up confessing to killing Christian Matthews and told detectives the sewer drain they could — and indeed did — find the gloves he was wearing, according to court records.

In his interview, Lipford also implicated his girlfriend in the planning of the murder, the records show. Text messages between the two appeared to confirm the assertion, according to court documents. At one point, Leaundra Matthews texted to her boyfriend that many of their neighbors had security cameras and stayed out late.

Lipford suggested arriving at 1 a.m.

“No one up. Ppl gotta wurk in da am,” he wrote, using text shorthand.

“I need him in dere so I can take em out,” he also wrote. “Get rdy.”

Leaundra Matthews asked if she was to pick up just him or him and his friend Daniel Howard, who she referred to by nickname, police said.

“I have to pick up Ghost too?” she wrote back, according to charging documents.

The new records indicated detectives spoke with several witnesses who were in the townhouse at the time of the murder or spoke with the twins afterward. Investigators say in charging documents that just before the alleged killers went to the basement, one or both twins went to check and see if Christian Matthews was asleep. After the murder, detectives allege, the twins drove Howard and Lipford to a friend’s house.

It is not clear why investigators did not move on Leaundra Matthews in 2017. The records do show that on April 19, 2017 — a month after the murder — they brought Howard in for questioning.

“During the interview,” detectives wrote, “Daniel Howard admitted to being with Tysean Lipford at a friend’s house before the murder and after the murder. Daniel Howard also stated he was at Christian Matthews’s house during the murder, but he later recanted the statement.”

With a trial date set in February, Howard’s case is further along than Leaundra and Lemae’s cases. At an earlier court hearing, he spoke about wanting to be released on bond.

“This incident happened [four] years ago. Within that time, I have moved my way from it as far as possible. I found a steady job. I was able to move out of my mother’s home,” he said, his voice choking. “I have a dog now that I take care of. And I don’t plan on doing anything to put myself in prison and or jail. If you was to let me leave on bond, I will assure you the next time you see me in court, I will be proving my innocence.”