Frightened customers at a crowded Chicago pizzeria dove for cover behind tables and chairs as a gunman opened a door and began firing inside, killing one teen and injuring two others, police and a restaurant employee said.

"People were panicking. Everybody jumped under tables and some people ran out the back," said Lorena Sanchez, a cashier at Pizza Nova on the city's West Side.

Around 50 people were in the pizzeria Sunday evening when a lone gunman flung open the door and started shooting, Sanchez said. The shooting lasted for at least several minutes, she said.

"The guy shot five or six times," she said, speaking by telephone from the restaurant, which remained open Sunday night. "(The gunman) got away."

No suspects were in custody and the motive in the attack was not clear, police said Sunday.

"It's possibly gang related. But that hasn't been determined," said police spokesman Hector Alfaro.

A 15-year-old boy was shot and killed, while two men -- ages 18 and 19 -- suffered gunshot wounds, said another police spokesman, John Mirabelli. The two injured teenagers were in stable condition, he said.