As Prince George’s police surrounded the apartment of a man they believed had brutally slain his mother and half brother in Fort Washington, the man tossed a backpack from his window, prosecutors said in court Monday. Inside that pack investigators found four guns — one of which, they said, was later linked to the slaying.

That gun — a 45-caliber Desert Eagle — is pivotal to prosecutors’ case against Quindell Mercer, who is on trial this week for first-degree murder in the August 2011 slayings.

Prosecutors said in opening statements Monday that Mercer used the gun as he shot and killed 20-year-old Frank Lamar Harris and 56-year-old Patricia Harris inside their Fort Washington home.

“Not killed,” Prince George’s Assistant State’s Attorney Aaron Meyers told jurors. “Executed.”

Alan D’Appolito, Mercer’s defense attorney, did not directly address the weapon in his opening argument, but he said jurors would have significant doubts about whether Mercer was the killer by trial’s end.

Though he did not suggest it directly, Appolito seemed to cast Patricia’s husband, Frank’s father, as a suspect, noting that he acknowledged he came home from work and saw Patricia and Frank Harris sitting on the couch apparently after they had been killed. Appolito said that the husband had “serious mental problems.”

Meyers himself told jurors that the husband saw the bodies and did not call police until the next morning. He said the husband did not immediately realize the two had been slain because there was very little blood and no other signs of a problem. He said Patricia still had keys in her hand and Frank — who went by Lamar — had a tax form on his chest, as if he had fallen asleep filling it out.

“He thought they were sleeping,” Meyers told jurors.

Prosecutors did not discuss a motive for the slaying in their opening statement. Instead, Meyers focused on the evidence connecting Mercer to the crime. He said investigators recovered Mercer’s DNA on a magazine for the 45-caliber Desert Eagle, and an ex-girlfriend told police she had given Mercer the bag police in which police found the weapon.

Mercer’s trial is expected to continue through the week.