The FBI, the federal Bureau of Alcohol Tobacco and Firearms, and the DEA have joined the manhunt for a man who killed Philadelphia officer Chuck Cassidy.

Cassidy, 54, walked into a Dunkin' Donuts about 10:30 a.m. Wednesday, interrupting a robbery in progress. The gunman shot him in the head, probably before he had a chance to realize what was going on, and took Cassidy's service pistol. Cassidy, a 25-year veteran of the department, died Thursday from his wounds.

The gunman is known to be a heavy-set blackman with a spider tattoo on his left hand. He wore a hooded jacket during the robbery, so there is no clear description of his face.

Philadelphia police have sent the robbery video to the FBI Academy in Quantico, Va., for enhancement. They hope that the enhanced images can be used to create a sketch of the suspect.

In the meantime, authorities are stopping and questioning heavy-set black men with spider tattoos on their left hands.

Police believe the gunman may have been involved in at least four previous robberies, including a Sept. 18 robbery at the same Dunkin' Donuts.

The Philadelphia Police Department has set up a trust fund for Cassidy's family.