Police today charged a third suspect in connection with the robbery and fatal shooting of a University of Chicago graduate student.

Benjamin Williams, 21 of 15100 block of Sunset Drive, Dolton, is charged with first-degree murder in the death of Amadou Cisse. Though Williams and a codefendent are charged with murder, authorities said they are still not certain who fired the shots that killed Cisse.

Williams is also charged with two counts of armed robbery, one count of attempted armed robbery and one count of aggravated discharge of a weapon. The charges stem from a series of South Side robberies that occurred around the time Cisse was attacked, Chicago Police Sgt. Eugene Mullins said.

Williams is scheduled to appear in bond court tomorrow.

Two teens have already been charged in Cisse's death.

Demetrius Warren, 17, of the 5800 block of South Michigan Avenue, is charged with three counts of armed robbery and one count of aggravated discharge of a firearm.

Eric Walker, 16, of the 7700 block of South Marshfield Ave. is also charged with murder, one count of attempted armed robbery, three counts of armed robbery and one count of aggravated discharge of a firearm.

Cisse, 28, was shot in the chest in front of his Hyde Park home about 1:30 a.m.  on Nov. 19. He was weeks away from earning a doctorate in chemistry.