The explosion at Harvard Medical School over the weekend that drew a large emergency response and rattled the campus appears to have been from a firework set by two young men who’d come into Boston to party on Halloween and had little apparent motive, authorities said Tuesday.

Logan David Patterson, 18, of Plymouth, and Dominick Frank Cardoza, 20, of Bourne, were both arrested by federal agents Tuesday morning at their homes. Each made an initial appearance in court on Tuesday and was released as they face a charge of conspiracy to damage by fire or explosive, which carries a sentence of up to five years in federal prison upon conviction.

“Setting off an explosive device inside a locker at an institution geared toward higher education is not some harmless college prank,” said FBI Bos

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