With the sun high over the Florida Straits separating the United States from Cuba, a speedboat with registration details from Florida encountered a Cuban border force vessel.
Gunshots began to crack over the blue Caribbean waters.
Four people were shot dead, seven others — including a Cuban patrol commander — were injured.
Within hours, the Cuban government would claim it had thwarted what may have become an act of terrorism being transported onto its shores from the US.
Here is what is known about the shooting and how it unfolded.
Cuba claims it stopped 'armed terrorist infiltration'
In a broadcast on a state media television channel on Wednesday, local time, the Cuban government labelled the speedboat's movement into its territorial waters as an act of "armed aggression".
It said the vessel was registered in Florida with the number FL7726SH and had been transporting 10 armed individuals when it was intercepted in the morning by five border force troops aboard another vessel.
[DW Location]Individuals on the speedboat were the first to open fire, which provoked the troops to retaliate, the Cuban government said.
The gun fight occurred about one nautical mile north-east of a channel of water known as "El Pino", 200 kilometres east of the capital Havana
Four people aboard the speedboat were fatally shot and six others injured. A Cuban patrol commander aboard the border force vessel was also injured.
The individuals on the speedboat had planned to conduct "an infiltration with terrorist purposes," the Cuban Interior Ministry said later in a statement.
"[The border force] seized assault rifles, handguns, homemade explosive Molotov cocktails, bulletproof vests, telescopic sights and camouflage uniforms."
The speedboat appeared to be a 24-foot (7.3 metre) Pro-Line motorboat built in 1981, The New York Times reported.
Speedboat's passengers were 'violent criminals'
The Cuban government said six men were initially detained at the scene of the shooting.
They were identified in the state media bulletin as Amijail Sánchez González, Leordan Enrique Cruz Gómez, Conrado Galindo Sariol, José Manuel Rodríguez Castelló, Cristian Ernesto Acosta Guevara and Roberto Azcorra Consuegra.
Another man, Michel Ortega Casanova, was identified as one of the four people fatally shot.
Investigations are ongoing to identify the remaining three, according to Cuban authorities.
All of the people who were aboard the speedboat were native Cubans living in the United States, and the majority were known for having a history of "criminal and violent activity".
Mr González and Mr Gómez were also on a national watch list of people wanted for "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," the Cuban Interior Ministry said.
Another man, Duniel Hernández Santos, was identified after being detained on the Cuban mainland.
Authorities claimed he travelled to Cuba from the US to "guarantee the reception of the armed infiltration" and confessed his role in the plot on Wednesday.
Mr Sariol was interviewed in June last year by Martí Noticias, a US-based news site that has long called for a change of government in Cuba, according to the Associated Press.
Mr Sariol was quoted as saying he wanted to support the struggles faced by Cubans, especially in the eastern part of the island "to achieve the freedom that is needed."
He said the protests in Cuba at that time were "not a spark that's going to be extinguished."
"The regime's leaders are crisscrossing Cuba, trying to mitigate what's coming very soon," he was quoted as saying.
"They know they're out of power, that they can't do anything about it, and they're looking for ways to prevent the protests from growing in other parts of the country.
Cuban exiles, who are largely concentrated in Miami, Florida, have long dreamed of overthrowing the Cuban government or seeing it fall.
In the past, some have plotted against the Cuban government established by the late revolutionary leader Fidel Castro, who died in 2016 at age 90.
Cuban exiles with support and financing from the US CIA carried out the failed Bay of Pigs invasion in 1961, an event that strengthened Castro while pushing him closer to his backers in the Soviet Union.
Other Cuban paramilitaries have attempted or carried out acts of sabotage in decades past.
Misael Ortega Casanova, the brother of Michel Ortega Casanova, told the Associated Press that he was mourning his brother's death, but said he fell into what he called an "obsessive and diabolical" quest for Cuba's freedom.
"Only us Cubans who have lived over there understand," he said.
He said his brother, who was a truck driver and an American citizen, lived for more than 20 years in the US and leaves behind his wife, his mother, two sisters — one of whom lives in Cuba — and a daughter who is pregnant.
"No-one knew," Mr Casanova said of his brother's plans.
"My mother is devastated.
"They became so obsessed that they didn't think about the consequences nor their own lives."
Mr Casanova said he did not recognise any of the names that the Cuban government released.
How has the US responded?
US Secretary of State Marco Rubio told reporters on Wednesday, local time, he was made aware of the incident and that the US was gathering its own information on the incident.
"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 while in Basseterre, St Kitts.
"Suffice it to say, it is highly unusual to see shootouts in open sea like that.
"It's not something that happens every day. It's something, frankly, that hasn't happened with Cuba in a very long time."
He said both the US Department of Homeland Security and the US Coast Guard were investigating.
"The majority of the facts being publicly reported are those by the information provided by the Cubans. We will verify that independently as we gather more information, and we'll be prepared to respond accordingly," he said.
"We're going to have our own information on this. We're going to figure out exactly what happened."
Mr Rubio also told reporters the incident was not related to a US government operation.
The shooting threatens to increase tensions between the US and Cuba, which have heightened in the wake of the US capture of Venezuelan President Nicolás Maduro in January.
Since that military operation, US President Donald Trump and administration officials have taken an increasingly aggressive stance toward Cuba, which had been largely kept economically afloat by Venezuela's oil.
The energy crisis Cuba has grappled with in recent years entered new extremes last month when Mr Trump signed an executive order to impose a tariff on any country that sells or provides oil to Cuba.
Mr Rubio said the Cuban government needed to make "dramatic reforms that open the space for both economic and eventually political freedom for the people of Cuba."
ABC/Wires
