CHICAGO (CBS) -- Investigators think more people were involved in the murder of a Chicago police officer than the two people now awaiting trial for it.

Police have put out a flyer reminding the public there's a $33,000 reward for information leading to the arrest and conviction of anyone connected with the murder of Police Officer Clifton Lewis in December.

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The 41-year old officer had been working a second job as a convenience store security guard when he was killed during a hold-up at Division and Austin in the Austin neighborhood on the West Side.

Two men were arrested and charged with killing Officer Lewis.

29-year old Tyrone Clay and 34-year old Edgardo Colon are being held without bond in Cook County Jail.

Police Lt. Maureen Biggane says the re-publishing of the reward flyer is being done to help detectives find at least one other person they believe is involved and to build a stronger case against Clay and Colon.