Authorities issued an arrest warrant on Sunday for a suspect in the slaying of a city police officer who interrupted an armed robbery at a doughnut shop.

Police Commissioner Sylvester Johnson identified the suspect as 21-year-old John Lewis, also known as Jordan Lewis, and urged anyone who might know his whereabouts to contact police. He also called on Lewis to turn himself in.

"He killed a police officer, he has at least two guns, he is armed and extremely dangerous," Johnson said.

Lewis can be identified by tattoos on his hands that read "N.P." and "H.P." for North Philadelphia and Hunting Park, two city neighborhoods he frequents, Johnson said.

Officer Chuck Cassidy, 54, died Thursday, a day after the married father of three was shot in the head as he walked in on a robbery at a Dunkin' Donuts in the city's West Oak Lane neighborhood.

A round-the-clock manhunt has been under way since the officer's shooting, as investigators blanketed the neighborhoods near the crime scene, questioning residents and bringing in about a half dozen people for questioning.

Police declined to release any information about the investigation, instead stressing the importance of getting Lewis into custody.

"We don't want anyone else to get hurt," Johnson said.

"My condolences again go out to the Cassidy family. We've talked to Lewis' family, they're also hurting. Everyone is hurting," he said. "We need nobody else in the city of Philadelphia to be hurt by this individual ... ."

Witnesses said the gunman went into the shop and demanded money just before the policeman opened the door, then spun around and shot Cassidy before the officer could react.

Portions of a chilling videotape released by police show a hooded robber pushing aside two customers and waving a gun as he approaches the counter. The tape also shows him grabbing the fallen officer's pistol as he flees.

"You could see on the films that he has no hesitation in shooting a person. If he could shoot a person in uniform just coming into a Dunkin' Donuts, he's liable to shoot anybody," Johnson said.

The shooting came about 12 hours after a masked gunman shot another officer in the shoulder during a chase downtown. That officer was released from the hospital Thursday.

Cassidy's death marked the second death of a city police officer in 18 months.

In May 2006, Officer Gary Skerski was fatally shot in the neck when a man robbing a bar fired a shotgun out the back door. The killer, Solomon Montgomery, pleaded guilty to first-degree murder and was sentenced Tuesday to life in prison.

More than $153,000 has been donated as a reward for finding Cassidy's killer, but Johnson downplayed the reward money.

"Money has no importance to us at this point," Johnson said. "The whole idea is to apprehend this male, get him into custody. We don't just want an arrest; we want a conviction."

A private viewing for Cassidy's family and close friends will take place Tuesday night at Givnish Funeral Home in Northeast Philadelphia. A public viewing and funeral service will be held downtown Wednesday at the Cathedral Basilica of Saints Peter and Paul.