It was a Sunday in September 2009, and Kareem Kelly Guest had spent the entire day assembling a bedroom for his 6-year-old daughter in his mother's Westport home. He'd been released from jail three weeks earlier and was making up for lost time.

He left the house after dark to pick up the little girl and show her his handiwork. But he never made it. Guest was executed shortly after stepping outside, shot twice in the back and four times in the head.

Guest had been labeled a rat, and everybody knew it, prosecutors said Tuesday in Baltimore's U.S. District Court, during opening statements in the federal trial of Antonio "Mack" Hall.

He's charged with brazenly killing Guest in retaliation for speaking out and accused of shooting a different witness, who survived. He's also charged with killing a fellow crack cocaine dealer. Witnesses against him are expected to include his friends, former lovers, victims and drug customers.

And they all have one thing in common, said Assistant U.S. Attorney John Purcell: They're frightened.

"They'd [rather] implicate their mothers or fathers before they'd implicate him," Purcell told jurors. "Every single one of them is scared of him."

It's a case that resonates in Baltimore, where the life of a witness has sometimes been at risk. In June 2009, shortly before Guest was killed, two men were convicted in federal court for the contract killing of Carl Lackl, who had been scheduled to testify in a murder case. City trials are routinely upset by claims of intimidation.

"The law of the street says you don't say anything," said Purcell, who also prosecuted the Lackl case. "Kareem violated the rule."

Guest named nearly 20 people as drug dealers, robbers and contract killers in an interview with the FBI, describing Hall as someone who likes to "bang the gun."

A transcript of that conversation was wallpapered around the Westport neighborhood after Michael R. Carithers Jr., a defense attorney who was recently disbarred for unrelated reasons, gave a client copies, which he admitted to in court on Tuesday.

In Baltimore, birthplace of the underground "Stop Snitching'" videos, that meant Guest was as good as dead, attorneys on both sides said.

Still, Hall's lawyers say someone else is to blame.

"A lot of people know Kareem was snitching, a lot of people had incentive to kill him," said Gary Proctor, one of Hall's court-appointed lawyers.

Most of the witnesses in the case changed their stories halfway through, Proctor said, only implicating Hall after the government ramped up the pressure. Some are being paid for their participation, he added, and one woman was indicted for perjury before she finally pointed to Hall.

"You've got nothing but liars, lying liars, perjurers, whatever you want to call them. That's what you've got," Proctor said.

The trial began Tuesday with Guest's mother taking the stand. She dabbed away tears as she described the plan to surprise her granddaughter with a new bedroom.

"That was the last time I saw him," she said. But people had been telling her for weeks that her son was in trouble.

"Everybody said Kareem was the rat. Kareem told on other people — the people in these papers — and sent them to jail," she said.

Guest was arrested on drug charges in 2008, and chose to cooperate with federal investigators. His interview contributed to the federal indictment of eight people on drug charges, most of whom pleaded guilty and avoided trial.