Parker is serving a 19-year prison term at the Federal Correctional Institute in Loretto, Pa., for bank robbery. He had previous weapons convictions.
Authorities said Williams was at the federal prison in Fort Dix, serving an eight-year sentence for drug and weapons charges.
The case against the pair was aided by their alleged accomplice in the 2008 killings, Tarik J. Mackins, 32, of Trenton, who was allowed to plead guilty in February to two counts of aggravated manslaughter and who agreed to cooperate with the murder investigation. He has not yet been sentenced, but he must serve at least 17 years.
The murders were carried out because of a Bloods threat to harm a Muslims Over Everything member, authorities said.
On Sept. 20, 2008, Darryl Sample, 23, a Willingboro Bloods member, was fatally shot once in the chest and once in a leg while walking toward Beverly-Rancocas Road in Willingboro about 9:30 p.m. Authorities allege that Parker and Williams were the shooters and that Mackins drove the getaway vehicle.
On Sept. 22, 2008, Deion Madison, 28, also a reputed Blood, was sitting in the driver's seat of his Lexus in the parking lot of a Charleston Road strip mall in Willingboro about 6:15 p.m. when he was approached by Mackins. Mackins reached through the open passenger-side window and fired several shots from a .357 Magnum handgun. Madison was struck five times in the abdomen, an elbow, and a thigh. He died four days later at Cooper University Hospital in Camden.
Parker and Williams allegedly planned Madison's murder and waited in a nearby vehicle to drive Mackins away.
The attempted-murder charge against Parker and Williams stems from an unsuccessful try at killing a Bloods gang member the day before Sample was gunned down. Authorities allege the pair fired on a car containing a Bloods member, but no one was hit.
At his 2010 robbery sentencing, Parker told U.S. District Judge Noel Hillman in Camden he never knew his father and his mother went to prison when he was 14.
"I played the hand that was dealt to me," he said.
"I've been around drugs and violence my whole life."
Contact Rita Giordano at 856-779-3893, firstname.lastname@example.org, or follow on Twitter @ritagiordano.