Cuba’s military killed four Cuban Americans aboard a U.S.-registered boat that entered Cuban waters Wednesday, calling it a “foiled armed infiltration” aimed at terrorist activities.

The Interior Ministry said the vessel carried 10 armed individuals with rifles, handguns, Molotov cocktails, and bulletproof gear, and that a firefight injured a Cuban border guard and six people on the boat.

Cuban President Miguel Díaz-Canel vowed to defend against any “terrorist and mercenary aggression,” while U.S. officials, including Senator Marco Rubio, announced an independent investigation.

The ministry said the boat’s crew opened fire near El Pino channel, and six people were detained; some reportedly have criminal records, NBC News has reported.

U.S. lawmakers from Florida have called for scrutiny of Cuba’s account, emphasizing the need to determine whether any U.S. residents were victims.