The two Good Samaritans, whose names have not been made public, hopped off a train in Brooklyn when they saw a bleeding man pummeling the officer, the New York Daily News reported Saturday.
The officer spotted 43-year-old David Robinson bleeding from his ankle and offered to help when he said Robinson whirled around and knocked the officer to the ground, fracturing his wrist.
One of the men assisting the officer radioed for help and the other slapped handcuffs on the suspect.
Robinson died about an hour and a half after the attack. He was treated at Interfaith Medical Center. He apparently went into cardiac arrest after suffering an asthma attack, police officials said.
The officer, whose name has not been made public was treated and released.
Police said Robinson had had 17 prior arrests and witnesses say he was seen at a burglary before the attack, and may have gone berserk when the officer approached him, fearing arrest.