It was a story Sir Jack Matthews had told to detectives, prosecutors, friends and special agents numerous times — how he and two friends kidnapped and killed an Atlanta restaurant owner.

But Wednesday morning — in front of a jury and to the surprise of prosecutors and defense attorneys — Mr. Matthews changed his story.

Testifying in the federal murder trial of Rejon Taylor, Mr. Matthews said he and another man conjured a story to place blame on Mr. Taylor if things “crashed down” after the killing.

Mr. Taylor is standing trial on murder charges in connection with Guy Luck’s death. He faces the death penalty if convicted in what is the first death-penalty trial in the history of Eastern Tennessee’s federal courts district.

Mr. Matthews, a key witness for the prosecution who pleaded guilty in 2006 to murder, kidnapping and carjacking in exchange for testifying against Mr. Taylor, told the court he shot Atlanta restaurant owner Guy Luck in the arm on Aug. 6, 2003, while he and Mr. Taylor were in Mr. Luck’s white cargo van. They were helping Mr. Luck move boxes, one of which contained marijuana, from Georgia to Tennessee, Mr. Matthews testified.

In testimony, Mr. Matthews said he shot Mr. Luck in the arm after the restaurant owner said he was going to kill Mr. Matthews and Mr. Taylor because he knew they had burglarized his home. The threat came, Mr. Matthews said, after he made a comment about pictures in a lockbox that had been stolen from Mr. Luck’s home.

Mr. Taylor, hearing the shot that hit Mr. Luck, then shot Mr. Matthews in the arm accidentally, according to Mr. Matthews’ testimony.

After Mr. Matthews testified, prosecutors this morning tried to show inconsistencies between his testimony versus what he told detectives in 2003. On a taped interview shown in court, Mr. Matthews told a Hamilton County Sheriff’s Department detective in 2003 that he shot Mr. Luck, got out of the van that he, Mr. Luck and Mr. Taylor were in and heard two to three more shots from inside the vehicle.

He repeated that story in a letter he wrote from prison to a friend that the prosecution presented as evidence.

On the stand Wednesday morning, Mr. Matthews said he did not hear any more shots after exiting the van.

Mr. Matthews said he was telling the truth for the first time Wednesday because he didn’t want to bear the burden of Mr. Taylor’s death should the defendant be found guilty and sentenced to death.

“I’m telling you it because I ain’t had peace since the situation,” Mr. Matthews said.

Assistant U.S. Attorney Steve Neff declined comment on Wednesday’s turn of events. Court will not be held today, but will be back in session at 9 a.m. Friday when the prosecution is expected to call Hamilton County Medical Examiner Dr. Frank King to testify.

Prosecutors have said that Mr. Taylor, Mr. Matthews and co-defendant Joey Marshall kidnapped Mr. Luck at his home in the Buckhead neighborhood of Atlanta and drove him to Collegedale, where they shot him several times on the side of the road. Mr. Luck died at Erlanger hospital.

Mr. Marshall, who also has pleaded guilty to murder, kidnapping and carjacking, testified last week.

A Tennessee Bureau of Investigation agent who specializes in fingerprint testing testified that half a palm print found in the back of the white van “without a shadow of a doubt” matched that of Mr. Taylor.

In his testimony Wednesday, Mr. Matthews also accused prosecutors and an FBI agent of telling him what to say when he appeared on the witness stand.

“Y’all gave me certain things to mix it up, make it look worse than it is,” he said.

In an attempt to contradict their witness, prosecutors asked FBI Special Agent James Melia whether, in any of his 10 or so interviews with Mr. Matthews, he or prosecutors instructed Mr. Matthews on what to say.

“We never gave him any words,” Agent Melia said. “We explored discrepancies in his stories ... We never asked him to say anything other than the truth.”

In another surprise, a jail search of Mr. Matthews’ cell Wednesday morning yielded an envelope with the words “Rejon Taylor,” “Leslie Cory, atty.” and “legal matters” written on it. Lt. Gene Coppinger with the Hamilton County Sheriff’s Department corrections division testified that Mr. Matthews and Mr. Taylor shared a common cell area but not the same sleeping cell.

Ms. Cory is one of the defense attorneys for Mr. Taylor.

The enveloped contained a transcript of Mr. Matthews’ interview with a Hamilton County Sheriff’s Department detective, the defense showed on cross-examination.

Prosecutors said conversations about legal matters between Mr. Matthews and Mr. Taylor might have led Mr. Matthews to change his story.

Defense attorneys disagreed.