WOBURN, Mass.—As the second week of the trial of alleged Kirkland shooter Jabrai Jordan Copney came to a close, an alleged cooperator in the crime was grilled on the witness stand by the defense.

Prosecutors have granted the witness, Blayn Jiggetts, a plea bargain in exchange for testifying. On the stand Friday, Jiggetts said that Copney planned to rob Justin Cosby, the victim, and fired the shot that killed him—but defense attorney John A. Amabile has said since the opening statements that Jiggetts was actually the man who pulled the trigger.

In court on Friday, Amabile spent nearly three hours dissecting Jiggetts’ version of the events in the case.

Amabile suggested that Jiggetts and Jason Aquino—the third man who was indicted for the murder and who has also reached a plea agreement—collaborated on a story which would pin Copney for the murder.

Amabile established in his questioning that Jiggetts and Jason Aquino were friends since childhood, whereas Copney had met them both within the year that the shooting took place.

After noting that Jiggetts has called Aquino “cousin” because of their close camaraderie, Amabile referred to Aquino as “cousin Jason” repeatedly during his cross-examination.

“We don’t know what happened; it’s all Jabrai Copney—that’s the story you and your pal, your cousin Jason Aquino talked about,” Amabile said. “You made an agreement that the two of you, who didn’t care what happened to Copney, would lie to the police.”

Jiggetts denied Amabile’s interpretation, saying that Copney himself concocted a cover story that the men originally told to investigators.

Jiggetts said on the stand that he sought out the gun, which was owned by Aquino, and brought it from New York City, the three men’s hometown. Jiggetts further admitted to loading the gun in a Harvard dorm room before the shooting. He said he pulled the gun in the Kirkland annex basement to threaten Cosby—the eventual shooting victim—before handing it over to Copney.

“Why did you bring bullets with you?” Amabile asked.

“I was told by Jabrai it was vital because we were dealing with a street kid,” Jiggetts said, referring to Cosby, the intended target of an armed robbery of marijuana which Jiggetts says the three men had planned.

Amabile continued, “You brought bullets because you wanted to be prepared to shoot somebody. Is that correct, sir?”

When Jiggetts replied, “Not at all,” Amabile came close to shouting.

“That’s the only reason you need bullets,” he said. “Is that correct?”

In the final line of questioning in his lengthy cross-examination, Amabile focused on the fact that in exchange for his cooperation against Copney, Jiggetts has been promised a 9 to 12 year prison sentence, two years of which he has already served—as opposed to the life sentence without parole which Copney could face for a first-degree murder conviction.

“You know darn right well that you’re going to get out seven years from now—and when you’re 28 years old, you’re going to be flying free, even though you killed somebody,” Amabile said.

“I didn’t kill anybody,” Jiggetts responded.

After Jiggetts left the witness stand moments after that exchange, Amabile cross-examined ballistics expert Stephen Walsh, who inspected the crime scene—Kirkland’s J-entryway—on the day of the shooting and the following day. During Amabile’s questioning, Walsh said that the bullet he found embedded in the wall of the landing on the way to the entryway’s basement seemed to have been fired from above, while the bullet lodged in the door of the entryway seemed to have been fired upward. The trajectories of the two shots, Walsh confirmed, suggests a shooter who was standing on the stairs above the landing.

As Jiggetts told it, Copney fired at Cosby while standing in the doorway of an art room in the basement, rather than on the staircase. A gunman in that position would have to fire bullets which could turn corners in order to reach the door, Amabile said, comparing such a feat to a Road Runner cartoon.

“That’s fake, isn’t it?” Amabile asked Walsh. “That defies the laws of gravity and physics—that’s only in a cartoon, isn’t it?”

After a break for Patriots’ Day today, the third week of testimony in the trial will commence tomorrow.

—Xi Yu contributed reporting to this story.

—Staff writer Julie M. Zauzmer can be reached at jzauzmer@college.harvard.edu.