Two young men from Massachusetts have been arrested in connection with Saturday’s explosion at Harvard Medical School, the FBI said.

Logan David Patterson, 18, and Dominick Frank Cardoza, 20, were taken into custody early Tuesday and charged for their alleged involvement in the explosion, which occurred on the fourth floor of the Goldenson Building just before 3 a.m., officials said.

The explosive, which detonated inside a research laboratory locker, was believed to be a Roman candle firework, according to Leah Belaire Foley, U.S. attorney for the District of Massachusetts.

No one was injured, officials said.

Patterson and Cardoza conspired to commit an “extremely dangerous act,” FBI Boston Special Agent in Charge Ted Docks said.

This is “not some harmless college prank,” Docks said. “It’s a federal crime,” he said, adding that there could’ve been casualties if people were in the building at the time.

Foley confirmed the suspects do not attend Harvard, but she noted there appeared to be “numerous parties ongoing on Friday.”

“This investigation is ongoing,” Docks said, adding, “There is no evidence of any ongoing threat to the university or to the public.”

Early Saturday, surveillance cameras captured two suspects in face coverings walking near the medical school and lighting what appeared to be Roman candle fireworks, Foley said. The suspects then allegedly climbed scaffolding and accessed the building’s roof, she said.

Shortly after, campus police were alerted to a fire alarm from an explosion on the fourth floor of the building, where there’s a research laboratory for the medical school’s department of neurobiology, Foley said.

Security video showed the two suspects leaving the building and fleeing the area in the opposite direction, Foley said.

The FBI recovered video allegedly showing Cardoza taking off his pants and throwing them in a garbage can after he fled the scene, Docks said. His pants were later retrieved by investigators, he said.

Members of the public called authorities when they recognized the suspects, Foley said.

“These two men allegedly boasted about what they did to their friends,” Docks noted.