SAN FRANCISCO (Reuters) - Police and federal agents converged on San Francisco General Hospital on Thursday to investigate reports of possible gunshots heard coming from the building, but found only the remnants of a detonated firecracker inside, authorities said.

San Francisco police Captain Daniel Perea said a sweep of the medical center on the edge of the city's Mission District confirmed there had been no shooting, and no one was injured.

Police said all that was found were debris from an M-80-type of explosive, a powerful firecracker commonly consisting of a cardboard tube fill with pyrotechnic flash powder and lit with a small fuse.

The hospital was not evacuated, but authorities sealed off the building during the sweep to prevent anyone from entering or leaving as the search progressed, police said.

Agents of the Federal Bureau of Investigation joined local police in the sweep of the hospital, according to Prentice Danner, a spokesman for the FBI in San Francisco.

(Reporting by Curtis Skinner in San Francisco, additional reporting by Alex Dobuzinskis in Los Angeles; Writing by Steve Gorman; Editing by G Crosse and Sandra Maler)