The executions of detectives James Nemorin and Rodney Andrews in Staten Island occurred suddenly, without warning, and were not part of any spoken plan, an eyewitness to the 2003 killings testified yesterday in federal court in Brooklyn yesterday.

The eyewitness, Jesse Jacobus, then 17, testified that at the time of the murders he was in the backseat of a Nissan Maxima sitting behind the two detectives, who were both undercover. The fourth man in the car, the accused killer, Ronell Wilson, also sat in the backseat, next to Jacobus.

The detectives, part of the Firearms Investigation Unit, had come to Staten Island to buy a Tec-9 pistol. Instead of the sale, there was an aimless drive that ended when Wilson drew a gun from his waistband and shot the two detectives in rapid succession, Jacobus testified.

It is Mr. Wilson who faces the death penalty if he is convicted of the racketeering crimes he is charged with. Jacobus is currently in prison awaiting sentencing by a state judge on the charge of second-degree murder that he pleaded guilty to in state court.

Jacobus’s physical proximity to the executions can be measured in inches. But the question of how close he stands to the crime itself will never be tried before a jury.

An assistant U.S. attorney, Jack Smith, asked Jacobus to state exactly what crime was contained in the guilty plea he has entered.

“Basically, guilty by association,” Jacobus, now 20, said.

The jury followed the testimony closely and declined an offer of an early afternoon break by Judge Nicholas Garaufis.

Jacobus described himself as a “stickup man” and hustler for the Stapleton Crew gang, based in the public projects of the Stapleton Houses on Staten Island. He was valued for his size. At 17 he already weighed more than 200 pounds and other gang members tasked him with physically snatching drugs from the pockets of rival dealers whom they were robbing at gunpoint.

“I was on the come-up,” Jacobus said, referring to his rising standing within the gang.

On March 10, 2003, the day Detective Nemorin had arranged to purchase a gun from the Stapleton Crew, gang members held an afternoon planning session. Jacobus left the meeting periodically to have a couple of drinks and smoke marijuana. Prosecutors have said that gang members had discussed the possibility that Nemorin was a detective, but Jacobus did not speak to that.

He did say that the plan was altered over the course of the afternoon.

“It changed from a gun sale to a robbery,” Jacobus testified. “I agreed to participate.”

Jacobus and Wilson had only been expecting one victim, Nemorin. But two men, Nemorin and Andrews, drove up at around 7p.m. For a moment it looked like the plan would be called off.

“Ronell had said we were going to walk away,” Jacobus testified.

And so Jacobus had started walking.

“Then he just said matter of fact we’re going to go on with it,” Jacobus said, referring to Mr. Wilson. And Jacobus got into the backseat. So did Mr. Wilson.