CUBA — Cuban authorities say their coast guard shot dead four people and wounded six others aboard a Florida-registered speedboat on Wednesday following an exchange of gunfire off Cuba’s northern coast.

According to a statement from Cuba’s Interior Ministry, the speedboat — detected about one nautical mile from Falcones Cay in Villa Clara Province — failed to stop when approached by a coast guard vessel trying to identify it. A passenger on the speedboat allegedly opened fire, injuring the commanding officer of the Cuban patrol boat, prompting return fire by Cuban forces.

The ministry said four individuals on the vessel were killed and six others were wounded. Those injured were evacuated and are receiving medical care. Cuban authorities later said the passengers were Cuban nationals living in the United States and were armed with rifles, handguns and Molotov cocktails, claiming they intended to “infiltrate” the island for terrorist purposes.

The identities and nationalities of the victims have not been independently confirmed, and U.S. officials are seeking their own facts about the incident. U.S. Secretary of State Marco Rubio said Washington is investigating and would “respond accordingly,” while Florida’s attorney general has also launched a probe into the shooting.

The confrontation comes amid already strained relations between the United States and Cuba, and is likely to heighten diplomatic tensions.