A heavily armed man who crashed his pickup truck on Long Island, then shot an emergency medical technician who went to the accident scene before being killed by the police, appeared to be on his way to carry out a mass killing, the police said Wednesday.

The dead man, whose name was not released by the police, had a rifle strapped to his chest and extra ammunition inserted in elongated wristbands on his arms, the Nassau County police commissioner, Lawrence W. Mulvey, said.

The man had six weapons, including a Tec-9 automatic pistol.

“It is clear to us that this man was out to commit mayhem in this county,” Mr. Mulvey said of the suspect, whom he described as about 31. The police believe he moved to Long Island from Florida in the past year.

The suspect had an arrest record for petty larceny, but none for any violent crimes, the police said.