Boston police officers dispatched to a Shore Plaza housing development in East ­Boston on Monday believed they were conducting a routine drug investigation but instead discovered high-powered weapons, including a grenade launcher and more than 2,500 rounds of ammunition.

The weapons were allegedly found in 22-year-old Michael Stroll’s car. On Tuesday, Stroll, of Randolph, was charged with unlawful possession of a firearm, unlawful possession of ammunition, unlawful possession of a large-capacity weapon, and operating a motor ­vehicle without a license.

According to the police ­report, officers from the A-7 station and neighborhood drug unit were investigating alleged drug activity at the Shore Plaza East apartments in the 700 block of Border Street on Monday night. Police said, “This specific development is known to officers for an abundance of drug-related activities.”

Around 8:17 p.m. police saw a white Subaru leave the development and pull into a parking lot of the development. Stroll and a 20-year-old male 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, who police say is 15 years old, got out and positioned himself as if he were a lookout.

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Police said the 20-year-old’s reaction, after he looked inside the trunk, sparked the officers’ interest.

“One of the individuals react[ed] in a startled, almost enthusiastic manner after inspect­ing the contents of the trunk,” police said in a posting online. “Specifically, officers observed the individual, while nodding emphatically, stagger back and throw his hands in the air in show of disbelief.”

After the men got back in the car and drove away, officers pulled them over for an improper signal. The 20-year-old told police that Stroll tried to sell him a Gucci belt and bag, but then admitted that Stroll showed him a bag of ­bullets, according to the police report.

In the trunk, police allegedly found a grenade launcher, two large brown boxes loaded with more than 1,000 rounds of various kinds of ammunition, a green military ammunition container with an additional 845 rounds, and a black handgun.

Stroll was arrested and held on $25,000 cash bail. Pretrial hearings begin July 13 in ­Boston Municipal Gun Court.

Jake Wark, a spokesman for Suffolk District Attorney ­Daniel F. Conley, said his office is working with police to determine the origin and intended destination of the weapons.

Peter Wu, who owns an ­auto repair shop across the street from the Shore Plaza development, said he was surprised to hear of the arrest, ­because the area has been relatively quiet in recent years.

“When I first moved my business here, there were used syringe needles on the sidewalks and cars had broken windows,” said Wu, who opened J & P Autobody on ­Border Street in 1997. “There have been a lot of improvements in the area.”