For the second time in two years, Los Angeles detectives have found a man dead inside the apartment of Ed Buck, a political activist who rose to national prominence in the late 1980s for his effort to recall the governor of Arizona, the authorities said.

The Los Angeles County Sheriff’s Department said it received a 911 call shortly after 1 a.m. on Monday from a person who claimed that a man inside a West Hollywood apartment had stopped breathing and could not be resuscitated. When emergency responders arrived at the residence, a second-floor unit on Laurel Avenue, they declared the man dead.

The authorities have not determined how the man died, nor have they publicly released his identity other than to say he was an adult black man. A group of protesters rallied outside Mr. Buck’s apartment throughout the day Monday and demanded justice for the man and for another black man, Gemmel Moore, who was found naked and dead of a drug overdose on a mattress in Mr. Buck’s living room in July 2017.

The sheriff’s office said that homicide detectives were among those who responded to Mr. Buck’s residence on Monday, and that it would review its investigation into the death of Mr. Moore, 26, and conduct new interviews. Mr. Buck, now 64, was not charged in Mr. Moore’s death.