Stephen Pitcairn got off the Bolt bus in Baltimore around 11 p.m. on July 25 last year, two days shy of his 24th birthday, and called his mother in Florida as he walked home from Penn Station, traveling north on St. Paul Street.

"I always feel so safe when you're on the phone with me," she remembers him saying that Sunday night.

They talked about the weekend, which he spent in New York City with his two sisters, and his plans to add a Saturday shift to his busy schedule as a Johns Hopkins cancer researcher. Then, "all of a sudden," his mother told a Baltimore jury Tuesday, "I heard him gasp."

Clutching tissues and what appeared to be rosary beads, Gwen Pitcairn tearfully recounted what happened afterward, when she heard her son's robbery and murder occur a thousand miles away over a cellphone connection. It was the first time she had spoken publicly about his death, and it made for an emotional start to the trial of a man charged with his killing.

John Alexander Wagner, 38, is accused of stabbing Stephen Pitcairn to death with his girlfriend, Lavelva Merritt, 25, so that they could get money for crack cocaine, prosecutors told the judge, shortly before the trial opened.

The case became a rallying point during last year's heated state's attorney's race, and a symbol of lax prosecution. Wagner has previously been convicted for theft, violence and repeated probation violations, but he's served little jail time, while Merritt has been found guilty of at least five prior drug crimes.

Baltimore State's Attorney Gregg Bernstein made it his mission to prosecute violent repeat offenders like Wagner, while campaigning to unseat his predecessor.

Initially, the case appeared simple, with early arrests and multiple witnesses claiming Wagner and Merritt confessed their involvement. But it was shown to be more complicated in court Tuesday.

Two of the witnesses were unable to see the actual crime, and several others — who lived with Wagner and Merritt in an apartment on Maryland Avenue — admitted to committing criminal acts connected to the killing.

Merritt pleaded guilty this spring to her role in the robbery and agreed to testify against Wagner in exchange for a 15-year prison term. She's currently incarcerated, as is a former roommate, Kevin Dwayne Cosby, who was convicted last month of assault in a separate case.

The defense said Cosby, 48, is the killer. He has admitted to illegally using one of Stephen Pitcairn's stolen credit cards, which he claims Wagner gave him, to get cash for drugs.

"The police simply bought Kevin Cosby's story hook, line and sinker," Assistant Public Defender Gregory Fischer said during his opening statement to the jury, claiming Cosby lied to cover his own tracks.

"John Wagner is nothing more than a scapegoat," Fischer said. "The state's case is built on the falsehoods and fabrications of crackheads and con artists."

Prosecutor Josh Felsen urged jurors to discount the others, however, and train their focus on the actions of one man: defendant John Wagner.

Wagner committed a "brutal, senseless crime," Felsen said. Stephen Pitcairn "was walking on a city street, not bothering anyone, talking to his mother, minding his own business, when he was attacked."

His mother said she had no idea what was happening.

She had spent the day painting her bedroom and was cleaning out brushes when her son called. They had talked briefly earlier in the day, while he was at the bus station in New York City, and he promised to ring her again when he arrived in Baltimore.

The first thing he said was "I'm sorry it's so late," Gwen Pitcairn told jurors, her countenance changing with each memory.

She smiled fondly when a prosecutor showed her Stephen's wallet, which she had given him for Christmas. But her face quickly fell when it was moved back into evidence — an apparent reminder that it had been recovered as part of the investigation.