The New York City police detective who fired the first shots in the 50-bullet barrage that killed Sean Bell in 2006 has been fired, and three others involved in the shooting are being forced to resign, law enforcement officials said on Friday.

The decision came after a Police Department administrative trial in the fall found that the detective, Gescard F. Isnora, had acted improperly in the shooting that killed Mr. Bell on what was supposed to have been his wedding day and that he should be fired.

“There was nothing in the record to warrant overturning the decision of the department’s trial judge,” Deputy Commissioner Paul J. Browne said on Friday night.

Law enforcement officials said word of Police Commissioner Raymond W. Kelly’s decision came late Friday. Detective Isnora, an 11-year veteran, will not collect a pension, one official said. “He loses everything,” the official said.