Boston police

Boston police allege this grenade launcher was found in the trunk of a car in East Boston on Monday.

By Alejandra Matos Globe Correspondent

Boston police officers conducting a drug investigation in East Boston made a startling find Monday — a grenade launcher, a pistol and 1,000 rounds of ammunition were allegedly stashed in the trunk of a car being driven by a Randolph man.

Michael Stroll, 22, is facing charges of unlawful possession of a firearm, unlawful possession of ammunition, unlawful possession of a large-capacity weapon, and operating a motor vehicle without a license.

Stroll was arraigned in East Boston Municipal Court today.

In a posting on their website, police said officers from the A-7 station and neighborhood drug unit were investigating alleged drug activity at the Shore Plaza housing development in the 700 block of Border Street on Monday night.

Around 8:17 p.m., police spotted a white Subaru pull out of the development and then pull over on Border Street. Two people got out of the car, and went to the trunk area where they discussed what police suspected was a business deal. As the apparent talks continued, a third man got out and positioned himself as if he were a lookout, police said.

Police said the reaction of one of the men after he took a look inside the trunk sparked the investigative interest.

“One of the individuals react[ed] in a startled, almost enthusiastic manner after inspecting the contents of the trunk,’’ police said in the posting. “Specifically, officers observed the individual, while nodding emphatically, stagger back and throw his hands in the air in show of disbelief. On the heels of the reaction, officers observed both parties shake hands and then re-enter the motor vehicle.’’