Boston police have arrested a man they allege tried to steal a Mercedes Benz from a dealership by claiming he had a hand grenade.

Police say a man showed up at Expressway Toyota on Wednesday and asked to test drive the car. Staff, describing the man as “nervous and fidgety,” insisted on accompanying him.

When they got back, the man produced what appeared to be a grenade from his pants pocket and told the sales person to “get back or I will blow this up.”

He fled when he heard staff calling police and was caught a short time later with a fake grenade.

The suspect, 27-year-old Carlos Guzman, was ordered to undergo a 20-day mental health evaluation after pleading not guilty to attempted armed robbery at his arraignment Thursday.