UPDATED 01/06/12 11:43 a.m.

CHICAGO (CBS) — Police are questioning three people in the murder of Chicago Police Officer Clifton Lewis, but there have been no charges and no one is officially in custody.

As CBS 2’s Susanna Song reports, police are interviewing three people at the Grand Central Area police headquarters, 5555 W. Grand Ave., in the murder of the officer on the night of Dec. 29. As of 11 a.m., no one had officially been taken into custody, and no one had been charged, police said.

LISTEN: WBBM Newsradio’s Bob Roberts reports

On record, police News Affairs would only say the case remains under investigation.

Lewis, an eight-year police veteran, was working a second job as a security guard at the M & M Quick Foods convenience store, 1201 N. Austin Blvd., when he was shot dead as two masked men tried to rob the store.

Initially on Thursday, law enforcement sources said there were suspects in custody, and at least one man had been charged. But police Supt. Garry McCarthy said this was not the case.

“I’m not going to talk about the investigation,” McCarthy said after Lewis’ funeral service on Thursday. “We’re going to get him – make no mistake. I have 13,000 in the Chicago Police Department to back that up.”

On Wednesday, teams of Chicago Police officers went door-to-door in the West Side’s Austin neighborhood looking for clues in hopes of apprehending the two masked men who robbed the store and took Lewis’ life.

Sources said Lewis had been hired following another robbery three months earlier.

The total reward for finding Lewis’ killer is now up to $34,000. It includes $21,000 from the Police Memorial Foundation, $11,000 from the Chicago Fraternal Order of Police, and $2,000 from several West Side churches, led by Rev. Ira Acree.