By Syma Chowdhry

PHILADELPHIA (CBS) – The search is on for the suspect police say shot an off-duty officer early Sunday morning in West Philadelphia.

This happened just after midnight in the 7600 block of 7300 Brentwood Road.

Police say 46-year-old officer Robert Penn was getting out of his car at his home when an unknown man approached him, uttered some words then shot him in the shoulder, with a bullet passing through his back.

Penn was taken to the Hospital of the University of Pennsylvania where he is listed in critical, but stable condition. He is expected to make a full recovery.

Police say Penn, a 24 year veteran of the force, had just finished his shift and changed out of his uniform before heading home.

The search is now underway for the shooter.

“We’re not clear about the motive at this particular time,” says Deputy Commissioner Richard Ross. “We’re still trying to glean that information from the investigation.

Ross said witnesses describe the suspect as 5’8 and was wearing all black — black coat, hat and shoes. They say this is possibly a blackmail situation.

Police have brought in several people for questioning, but they are not considered suspects at this time.