Eleven days before Officer Chuck Cassidy was shot and killed, suspect John Lewis was identified to police as the gunman who held up a Feltonville pizza shop, but no arrest warrant was obtained.

After learning that from an Inquirer reporter yesterday, Police Commissioner Sylvester M. Johnson ordered an Internal Affairs investigation to determine why the detective assigned to the case never got that warrant.

Had a warrant been issued, Lewis might have been in custody before Oct. 31 - the day Cassidy was shot when he interrupted the gunman holding up a West Oak Lane doughnut shop. Cassidy died the next day.

"If we made no attempt to apprehend him, that's inexcusable," Johnson said. "It's very disturbing. But this still doesn't mean that it would have prevented Officer Cassidy from being shot. "

In another development, police confirmed they had questioned Lewis shortly after the killing but released him because they had been focused on another suspect.

On Oct. 20, an employee of Oasis Pizza identified Lewis - a regular customer - as the gunman who had held up the eatery that night.

"If they had caught the guy," the murder of Cassidy "would never have happened," the 21-year-old worker, who identified herself only as Melendez, said last night. "They left me with the impression that they knew where he was at, but they never caught him. "

Police would not identify the assigned detective or her supervisor, citing the internal investigation. Those familiar with the case say the detective, an experienced veteran, is distraught over Cassidy 's death.

Six days into a massive manhunt, Lewis, 21, an Olney High School dropout, was captured in Miami and confessed. He has been brought back here and is facing murder charges.

Internal Affairs investigators are reviewing exactly what happened in the weeks before the fatal robbery and whether Lewis should have been off the streets.

Yesterday, after learning of the pizza-shop robbery, Johnson consulted top commanders, including Chief of Detectives Keith Sadler and Deputy Commissioner Patricia Fox, before ordering the Internal Affairs probe.

Johnson said no disciplinary action, if warranted, would be taken until the investigation was completed. The detective did act quickly to identify a suspect, he said. She had discretion on how to proceed, he said, but it remains unclear exactly what she did.

Said Sadler: "At that point, the detectives are obligated to take the case forward. Something was not done. "

In the Oasis Pizza robbery, which occurred on a Saturday night, a male in a black hooded sweatshirt entered and demanded cash. He escaped with $150 from the shop, in the 4500 block of North Fifth Street. The employee told police that the robber was a regular who stopped by Tuesday nights.

The detective took the employee to Police Headquarters, at Eighth and Race Streets, and she combed through 641 mug shots on a computer. She identified Lewis as the gunman, police said.

The detective next put Lewis' mug shot in a photo lineup of criminals with similar appearances. Again, the employee picked him out, and signed a copy of the lineup, police said.

Last night, the employee said she had spoken with several detectives but could not recall their names.

"They told me they were going to get him, and evidently he didn't get caught," she said.

About a week after making the mug-shot ID, she said, investigators called to ask her to confirm the identification.

"They said they were making sure it was the guy," she said.

And that, she said, was the last she heard from police until Lewis was arrested in Miami. The next day, she said, investigators called and asked "when was the last time I saw the gentleman who robbed the store. "

"I told them, 'At home on my TV, watching the news. ' "

Sadler and Johnson said that even with a warrant, police might not have been able to apprehend Lewis, who regularly moved around, staying with relatives in the area of the robbed Dunkin' Donuts.

Johnson also noted that 10,000 warrants are out for people wanted in the city.

Meanwhile, another twist has emerged: Shortly after the shooting, Lewis' mother, a corrections officer, contacted police and said she thought her son was the gunman. At that time, there was no indication that Lewis was wanted for anything else.

Two days later, a man claiming to be the getaway driver in the Cassidy shooting identified a different suspect as the killer, and police thought they were close to an arrest.

Homicide investigators instructed authorities in the field, who had apprehended Lewis by then, to make a detailed report and forward it to the task force assigned to find Cassidy 's killer.

"They did the right thing," Johnson said. "They did a detailed 48" - a police report.

When police interviewed Lewis, he held out his hands to show that they did not have a tattoo of a spider or a spiderweb, as described by witnesses. He did have tattoos that read "HP" and "NP," for Hunting Park and North Philadelphia.

Lewis was let go.

Afterward, authorities realized the self-professed getaway driver was spinning a tale and charged him with obstruction of justice.

Authorities went back to other possible suspects. That was when Lewis' name surfaced again, from a tipster. On Nov. 3, police rushed to his mother's house, but missed Lewis by 30 minutes.

"We had a window of opportunity to catch him before he was on a bus to Miami," Sadler said.

That was where police got Lewis, on Nov. 6, after he checked into a homeless shelter under a fake name.

He admitted that he killed Cassidy . Then he apologized to the officer's family and said, "I never meant anything to happen like this. "

Contact staff writer Barbara Boyer at 215-313-3004 or bboyer@phillynews.com.