ERIE -- One of Pennsylvania's strangest criminal cases, a bank robbery carried out by a pizza delivery man with a bomb locked around his neck, came into clearer focus yesterday when one of the four suspects pleaded guilty.

Kenneth E. Barnes, 54, admitted he was part of a middle-aged gang that robbed a PNC Bank in Erie five years ago after forcing one of its own members to wear a collar equipped with a bomb.

Mr. Barnes, a short, stocky man with thick glasses, did not spell out whether he made the bomb or forced pizza delivery man Brian Wells to wear it during the robbery. Rather, Mr. Barnes simply confessed to being part of the conspiracy and abetting in the use of the deadly collar bomb.

Mr. Wells, who was 46 years old, walked into the bank wearing the bomb on Aug. 28, 2003. He passed a note to tellers, who were sufficiently terrified of the bomb he wore to hand over cash from their drawers. He made it out the door with about $8,700, but police soon caught him with the stolen money in his vehicle.

Mr. Wells then spun an elaborate story in which he claimed to be a victim. He said a group of black men had taken him hostage, forced him to put on the collar bomb and then rob the bank. He said he followed orders because he feared they would kill him.

Police found the bomb to be quite intricate and could not remove it from Mr. Wells' neck. They called for detonation experts, but the bomb exploded minutes later, killing Mr. Wells as he sat under police guard.

Assistant U.S. Attorney Marshall Piccinini, chief of the Erie division, said no black men were involved in the robbery or use of the bomb. He said Mr. Wells lied about that and numerous other details. The pizza delivery man was deeply involved in the crime, having met with his co-conspirators at least twice, including the night before the bank heist, Mr. Piccinini said.

Mr. Barnes' lawyers say he was not the mastermind, but a willing participant in the robbery. He faces 30 years to life in prison under the terms of his plea agreement with prosecutors, but hopes his cooperation in solving the case will lead to a lighter penalty. U.S. District Judge Sean J. McLaughlin will sentence Mr. Barnes on Dec. 3.

U.S. Attorney Mary Beth Buchanan identified the other two suspects in the bank robbery as Marjorie Diehl-Armstrong, 59, and her boyfriend, the late William Rothstein. Mr. Rothstein died of cancer after being indicted.

Ms. Diehl-Armstrong is in state prison, serving seven to 20 years for killing another of her boyfriends, James Roden. She pleaded guilty but mentally ill in Mr. Roden's murder. Prosecutors said Mr. Roden had threatened to expose the bank robbery plot, so she killed him.

Judge McLaughlin ruled in July that Ms. Diehl-Armstrong was not competent to stand trial in federal court in the robbery and death of Mr. Wells. She is to be re-evaluated so that another report on her mental condition can be considered by the judge this fall.

With Ms. Diehl-Armstrong in legal limbo, Mr. Barnes' confession helped tear open the case.

On the day of the bank robbery, Mr. Barnes said, he drove to a gas station with Ms. Diehl-Armstrong. Mr. Rothstein met them there. Prosecutors believe Mr. Rothstein then used a pay phone to call Mama Mia's Pizzeria, where he placed an order for delivery to a remote location. The delivery man, of course, was Mr. Wells, also part of the robbery gang, according to Mr. Barnes.

They met near a television tower. By Mr. Barnes' account, the pizza man became frightened when he realized that his cohorts expected him to wear a real bomb when he robbed the bank. At that point, he did not want to go through with the robbery.

Prosecutors say one of the others, possibly Mr. Rothstein, fired a round from a revolver. This gesture apparently was sufficient to intimidate Mr. Wells into going through with the bank robbery.

At a news conference yesterday, Ms. Buchanan would not answer questions about who built the bomb or who locked it around Mr. Wells' neck. She said she had to be guarded because a trial with Ms. Diehl-Armstrong as the defendant remains possible.

If Ms. Diehl-Armstrong does end up court, Mr. Barnes could be the star witness against her. Jamie Mead, a lawyer for Mr. Barnes, yesterday said his client could seek a reduction in his sentence based on his cooperation with law officers and prosecutors.

Ms. Buchanan said Mr. Barnes began divulging details of the crime as long ago as 2005. But, she said, he initially portrayed himself as an informant, not one of the robbers.

Still, his decision to cooperate is "a significant step toward closure to one of the most bizarre crimes ever committed" in Western Pennsylvania, she said.

Mr. Barnes said the bank robbery was supposed to finance another heinous crime. He said Ms. Diehl-Armstrong wanted her father dead, and he portrayed himself as a hit man. He joked that his fee for murder would be $200,000. The bank robbery was supposed to help pay the bill.