When D.C. police charged Joshua Holifield on Wednesday with killing a teenager in the Lincoln Heights area of Northeast Washington in the summer of 2007, he was already in prison, serving more than two decades for two other violent deaths.

The new case is the third fatal shooting in a single month that police say the 24-year-old from Suitland is responsible for, all stemming from disputes over the sale of guns. If convicted, Holifield could remain behind bars far beyond his expected release date in 2028. He was 17 at the time of the killings.

The latest charge filed against Holifield — first-degree murder — alleges that on June 2, 2007, he shot Terrence Fowler, 19, of Upper Marlboro eight times over $400 owed for a gun. The shooting occurred on Clay Street.

Less than four weeks later, on June 28, Holifield and his cousin Marcus Whitaker, 23, met two people in Southeast who wanted to buy Holifield’s five-shot revolver, police said. When the deal went bad, Whitaker and 17-year-old William Newman were killed. Holifield and Whitaker had tried to rob Newman and the other man. Authorities said Holifield fired first, striking Newman three times in the back and once in the arm. Newman’s friend pulled out a gun and fired once, striking Whitaker in the hip, police said.

Holifield was quickly charged in those shootings and pleaded guilty to two counts of manslaughter. A D.C. Superior Court judge sentenced him to 23 years in prison.

In 2008, an assistant U.S. attorney argued in court that Holifield “alone is responsible for initiating the shooting . . . that ultimately claimed the lives of both victims.” The prosecutor wrote in court documents that Holifield “provoked the violence.”

Relatives of the victims could not be reached Thursday.

In a sentencing memorandum, prosecutors noted Newman’s young age and the fact that he was an only son who lived with his mother, stepfather and stepsister in Temple Hills. He also was a junior at Crossland High School and had completed a educational workshop at Towson University four days before his death. Prosecutors included a poem Newman had written to his mother on Father’s Day, saying she filled the void in the absence of his biological father.

Holifield’s parents also could not be reached Thursday. Holi­field’s attorney, Santha Sonenberg of the D.C. Public Defender Service, argued in court in 2008 that her client should be sentenced as a juvenile and that his sentence should be lessened because the victims were “willing participants” in the gun transaction. She also noted that Holifield didn’t pull the trigger in his cousin’s death.

Sonenberg said that Holifield had come to Washington from North Carolina to search for his father and hooked up with friends and an uncle “who were older and who were a negative influence on him.” Court documents say that uncle, now dead, was Tyrone “Forty Cal” Hines, a nickname derived from a weapon.

Sonenberg, who represented Holifield in the manslaughter case, declined to comment.

Jennifer Jenkins contributed to this report.