The 17-year-old defendant kept his head down yesterday, chin in hand, as courtroom spectators watched a silent eight-minute surveillance video of an armed assailant holding up and fatally shooting a convenience store clerk in Mount Oliver.

The prosecution contends the masked, hooded robber in the video was defendant Eric D. Hancock, then 16, a North Carolina native who was visiting relatives in Mount Oliver last summer.

After viewing the video and hearing from a psychological expert, Allegheny County Judge Jeffrey A. Manning ruled that Mr. Hancock would be tried as an adult on charges of homicide, robbery and two gun violations. The trial also began yesterday.

According to the time stamp on the video images, the assailant entered the empty store at 8:56 a.m. Aug. 26. The intruder then hopped onto and over the counter and began waving a handgun at the man behind the register, who put his hands up. They appeared to talk, and the clerk gently batted away the muzzle. After four minutes, the assailant fired and the clerk clutched his chest and slumped to the floor.

The robber spent the next four minutes rifling through the register, trying keys for the cashbox, grabbing cigarettes and then fleeing, stuffing the cashbox inside a hooded sweatshirt.

A regular customer at A&E Deli Food Mart testified in the ensuing homicide trial that he came into the shop a few minutes later and phoned 911 after he saw Jamal Mouzaffar lying on the floor behind the counter, and thought Mr. Mouzaffar was having a seizure.

Mr. Mouzaffar, who had come to the United States from Syria to make adjustments to a prosthetic leg he'd gotten here, was helping out his uncle Abdul at the family store.

The 28-year-old amputee was a fourth-year journalism student in Damascus, and his uncle said he was extremely gracious to strangers. Abdul Mouzaffar had warned him not to put up a fight if someone held up the store. In the surveillance video, he appeared compliant.

Three of the defendant's cousins helped back the prosecution's theory that Mr. Hancock was the shooter.

Bethany Hancock testified, "I heard my cousin on the phone [afterward]. He said he had robbed a store and the dude died." Defense attorney Veronica Brestensky asked whether she shared that information with police after learning that her brother Jeremy was a possible suspect. She said yes.

Her younger brother, Justin D. Hancock, 17, told the judge he knew Eric Hancock wanted "to get a little money in his pocket" the night before the shooting.

"I told him to call his mom or wait till I got my paycheck. Don't do nothing stupid," he said.

Jeremy Hancock testified he heard Eric say that he wanted to get "hot licks," which meant "rob people."

The next night, Justin said, the defendant had more than $100 of cash and while the two cousins were playing video games, he said: "I did it."

A few days later, Eric Hancock turned himself in to police and gave a statement implicating a relative.

Testimony resumes today.