Raine Curtis reluctantly took the stand in federal court Wednesday and identified her former Baltimore boyfriend as a crack-dealing killer who carried guns and routinely wore a mask shoved back on his head like a hat — always accessible when he needed it.

Federal prosecutors say Antonio "Mack" Hall, 30, pulled down that mask to cover his face on the night of Sept. 20, 2009, and shot an FBI informant twice from behind as the man ran — snapping his spine — and four more times after he fell to the ground already dead.

"No, Mack, no," were the victim's last words, they said.

Curtis backed up the prosecutors' case Wednesday during the second day of Hall's trial in U.S. District Court — even though she had lied about it twice before to authorities and was indicted for perjury. She was taken away from her two children and held in a jail cell for more than half a year, released only after Hall was arrested in December.

"I felt that the importance of being with my children outweighed the importance of protecting someone," she told jurors Wednesday, explaining why she changed her story to implicate Hall.

She's one of at least a half-dozen people who switched from saying nothing to supporting the government's case, offering a rare glimpse of the strategy behind the difficult investigation. No one wants to come forward and be the witness in a murder case, prosecutors said, so they had to "break" them.

Some of the witnesses against Hall, including former attorney Michael R. Carithers Jr., were swayed by threats of criminal prosecution. Prosecutors say he released a copy of an FBI interview that showed Kareem Kelly Guest was a federal informant, leading to his death. Carithers originally denied releasing the document and is now under investigation for allegedly false statements.

Others were promised financial support through "witness protection expenses," such as rent. More than $26,000 has been paid on behalf of Curtis, who was relocated after she was released from jail. She said Wednesday that Hall implied he would kill her if she snitched.

Curtis was the first to talk, Assistant U.S. Attorney John Purcell said during opening statements this week, "not because she wanted to, because we broke her."

Prosecutors portray the tactics as skillful and necessary — giving in to lesser evils for the seemingly greater good of catching Hall, who's charged with killing Guest, 29, and with drug and weapons crimes. He's also accused in court papers of killing a teenage crack dealer and of shooting another witness in a separate incident.

But defense attorneys said the prosecutors' tactics seem more like bribery.

"The reason you talked to the government was not to be a good citizen," Gary Proctor, one of Hall's court-appointed attorneys, said to Curtis. "The reason you talked to the government was to get a better deal and get home, right?"

Curtis answered that she had told the truth.

Before Curtis, there was no case.

Months passed, and the murder case grew colder and closer to becoming just another unsolved city homicide. Everyone investigators interviewed — including Curtis — swore they saw nothing.

In February 2010, five months after Guest's death, a tip reinvigorated the investigation. Detectives learned that a call had been placed to Curtis the day after the killing from a prison phone, which meant the conversation was recorded.

A tape of the call was played in court Wednesday. "Kareem got killed last night. They killed that boy out there while we was right there …" Curtis says. "We was standing right there."

Officers and agents gave her another chance to change her story after that. But she refused, and was indicted for lying to a grand jury. She was held in jail for months, until not seeing her kids wore her down. She changed her claims in July of last year and was released from jail in December.

"This is a woman with some babies," Purcell said during opening statements, adding that the laws of motherhood trump the no-snitching laws of the street.

Curtis pleaded guilty in the perjury case and is awaiting sentencing, under the expectation that her cooperation could lead to a lighter prison term, if any.