US Twitter users are questioning the official account of the shooting of a black Muslim man by law enforcement on Tuesday.

Boston's police commissioner told reporters on Tuesday that Usaama Rahim had been shot and killed earlier in the day by a police officer and an FBI agent. Authorities say Rahim was under surveillance by terrorism investigators and threatened the officers with a military-style knife when they tried to question him.

Officers reportedly opened fire on Rahim when he refused orders to drop his weapon.

Many online shared a Facebook post from a man identifying as Rahim's brother, Ibrahim Rahim, a local imam.