Associates of one of the passengers killed in the alleged raid attempt said he sought to overthrow Cuba’s government.

Cuban authorities have accused 10 men on a speedboat sailing from the United States of attempting to carry out a campaign of violence to destabilise the island’s government.

Four of the 10 men on the boat were killed in a shootout with Cuban border agents this week.

On Thursday, Cuban authorities confirmed that the six remaining crew members were receiving medical treatment for their injuries. At least one Cuban officer was also hurt in the gunfire.

CBS News also reported that at least one US citizen was among the dead, citing White House officials.

Still, Cuban officials defended their government’s response and alleged the speedboat passengers shot first at border agents.

“Cuba will defend itself with determination and firmness against any terrorist and mercenary aggression that seeks to affect its sovereignty and national stability,” Cuban President Manuel Diaz-Canel said in a social media post.

The incident comes during a period of heightened tensions between Cuba and the US, which has made repeated threats against the island’s communist government.

The administration of President Donald Trump has also ratcheted up measures meant to worsen economic conditions there, including by imposing a fuel blockade on Cuba in late January.

Only one of the four men killed has been identified by the Cuban government so far: Michel Ortega Casanova.

One of Casanova’s associates told the news service AFP that he sought to overthrow the government in Havana.

“His goal was to go and fight against a criminal and murderous narco-tyrannical [government], to see if that would spark the people to rise up,” said Wilfredo Beyra, head of the Cuban Republican Party in Tampa.

Family members of Casanova said that they were not aware of his plans but that he was motivated by the “great suffering” of people under the Cuban government.

“No one knew,” Misrael Ortega Casanova, his brother, said. “My mother is devastated.”

He identified his sibling as part of a group concerned about rights abuses on the island.

“They became so obsessed that they didn’t think about the consequences nor their own lives,” Misrael added.

US officials have said that they will investigate the incident and denied any government involvement. Secretary of State Marco Rubio said that the US embassy in Havana is seeking more details, including whether US citizens are among those detained.

“We have various different elements of the US government that are trying to identify elements of the story that may not be provided to us now,” he said.

The US has worked with Cuban exiles against the country’s government in the past, including by supporting campaigns of clandestine infiltration, violence and sabotage.

But activist groups in South Florida have also emerged, some of them running small speedboats and planes to and from the island to ferry out Cuban nationals.

Cuban authorities have used US intervention as a pretext for the country’s steep restrictions on political dissent and a slate of security measures that rights groups say have been wielded against critics.

Cuban authorities identified seven of the 10 passengers, including Conrado Galindo Sariol, Jose Manuel Rodriguez Castello, Cristian Ernesto Acosta Guevara and Roberto Azcorra Consuegra. Cuba’s government said that one of the four killed was Michel Ortega Casanova. Three more have yet to be identified.

The Cuban government also stated that two of the passengers, Amijail Sanchez Gonzalez and Leordan Enrique Cruz Gomez, were wanted by law enforcement authorities “based on their involvement in the promotion, planning, organisation, financing, support or commission of actions carried out in the national territory or in other countries, in connection with acts of terrorism”.