In a pivotal day of testimony in the Phylicia Barnes murder trial, a 36-year-old petty criminal the defense has dismissed as a “jailhouse snitch” took the stand as one of the prosecution’s key witnesses.

James McCray, locked up in Charles County on a theft conviction, began his testimony by saying his name was actually Jason Lee and that he had drifted to Baltimore six years ago from New York and run an under-the-table business helping unqualified drivers get tags and titles.

McCray told jurors the defendant in the 16-year-old’s killing, Michael Maurice Johnson, called him for help disposing of a dead body.

“You won’t believe what happened,” he said Johnson told him as they stood over the North Carolina teen’s body wrapped in a bedsheet.

Phylicia Barnes, who lived in Monroe, was visiting relatives in Baltimore when she disappeared.

McCray’s testimony could be crucial to the case. Prosecutors cannot tell jurors conclusively how Phylicia died and there is a lack of forensic evidence. They also can’t place Johnson near the Susquehanna River, where her body was later found. He is the last known person to see her alive, but he can largely account for his whereabouts and actions that day, and detectives have testified that he cooperated with their investigation.

But if jurors believe McCray, that could help to overcome those gaps in the prosecution’s case.

Defense attorney Russell Neverdon denounced McCray as someone who “couldn’t tell Michael Johnson from Michael Jordan.”

Neverdon spent little time cross-examining McCray, shouting “no more questions” after McCray incorrectly testified that the killing took place before Christmas. Given a chance to help their witness clarify his statement, prosecutors also declined to ask more questions.

Prosecutors have said that McCray’s criminal record is minor. He also has testified in previous high-profile trials. In 2006, he helped Virginia prosecutors convict a 22-year-old man for a murder in the Old Town section of Alexandria.

The shooter, Sebastian Carter, confided in McCray while the two were inmates and said he planned to have a 13-year-old girl killed who was a witness to the crime, prompting McCray to come forward, according to news accounts. Carter, who wrote a confessional letter to his girlfriend, received 48 years in prison.

McCray also said on the stand Wednesday that he once testified in a Montgomery County rape case involving a teenage female victim. He said he testified reluctantly in each case.

He didn’t come forward with information about Phylicia’s death until June 2012, when he said he was overcome by pangs of guilt while reading the Bible in jail. Johnson had been indicted and charged in Phylicia’s death two months earlier, though McCray said he was not aware of that.

“I was feeling kinda bad about having information with respect to the young child’s murder, and not doing what I can to help out,” McCray said. He’s due to be released from jail soon and said he has nothing to gain by testifying.

According to McCray, he had connected with Johnson a few times through his underground business but stressed that he did not know him. He also said he had met Phylicia – who visited Baltimore only a handful of times – and said she stuck in his head because she had remembered him, calling him by the nickname of “New York.”

He said Johnson called him one day, explaining he had a “serious matter” and giving McCray directions to his apartment. Johnson took him into a bedroom, where McCray said it looked as if there had been a struggle. He said Johnson explained that “the girl had been giving him mixed signals about what direction they were going” and that Johnson forced himself on her.

“After that, she wouldn’t stop crying. He couldn’t get her to stop crying, so he killed her. He said he choked her,” McCray said.

McCray said he declined Johnson’s request to help him move the body but offered him advice based on a similar experience with someone else he knew. He said he told Johnson to turn off his phone so that his GPS data wouldn’t track his movements, and said that Johnson shouldn’t bury the body but toss it into water so that evidence they had had sex would be destroyed.

On cross-examination, Neverdon asked McCray if he had any mental conditions or had been held in a psychiatric facility; he said no.

Earlier in the day, assistant state medical examiner Pamela E. Southall testified that she concluded Phylicia was killed by asphyxiation. Southall said an examination of the body didn’t reveal any trauma and that asphyxiation was the default finding.

Asphyxiation encompasses a few types of death, including suffocation, strangulation, drowning, and compression. Southall said she couldn’t be sure which of those caused Phylicia’s death.

To make the finding of homicide, Southall said, she relied heavily on police investigators and their theories of the case. “The suspicious nature could not be overlooked,” Southall said. “You can’t ignore that she’s 16 years old, visiting from out of state, and ups and disappears.”

Neverdon questioned the logic, saying that was a theory and that the evidence suggested a range of possibilities, including that Phylicia could have drowned because of her own actions.

But Southall also said that the “moderate” decomposition of the body bolstered prosecutors’ theory that it was dumped into the water while inside a plastic tub, possibly helping to preserve the body for a time. “If not contained, you would expect more injury,” she said.