The United States is preparing to seek a criminal indictment against former Cuban President Raul Castro over the 1996 downing of two civilian aircraft operated by the humanitarian group Brothers to the Rescue, U.S. officials familiar with the matter told CBS News on Friday.

An indictment would mark one of the most significant escalations in U.S.-Cuba tensions in decades and could further strain already fragile diplomatic relations between Washington and Havana, though it would require approval from a grand jury.

The case centers on the Feb. 24, 1996, destruction of two unarmed Cessna aircraft flown by Brothers to the Rescue, a Miami-based exile organization that searched for Cuban migrants attempting to flee the island by raft and also supported dissidents opposed to Cuba's communist government. Four men were killed in the attack: Armando Alejandre Jr., Carlos Costa, Mario Manuel de la Pena, and Pablo Morales. Three were U.S. citizens, and Morales was a legal U.S. resident.

An investigation by the Organization of American States concluded the aircraft were shot down in international airspace and that Cuba violated international law by firing "without warning and without evidence that it was necessary." Then-President Bill Clinton condemned the attack "in the strongest possible terms."

Cuban officials have long defended the operation, arguing that the aircraft repeatedly violated Cuban airspace and were tied to anti-government activities aimed at destabilizing the island. Fidel Castro was Cuba's leader at the time, while Raul Castro served as defense minister and oversaw the armed forces.

The Justice Department declined to comment publicly on the possible indictment.

The development comes as the Trump administration intensifies pressure on Cuba through expanded sanctions and oil-related restrictions that have worsened fuel shortages and power outages across the island.

CIA Director John Ratcliffe traveled to Havana on Thursday for rare high-level talks with Cuban officials, including Raul Castro's grandson, Raul Guillermo Rodriguez Castro, known as "Raulito," Interior Minister Lazaro Alvarez Casas, and senior intelligence officials.

During the meetings, Ratcliffe delivered President Donald Trump's message that the United States is "prepared to seriously engage on economic and security issues, but only if Cuba makes fundamental changes," according to a CIA official. The official added that Cuba can "no longer be a safe haven for adversaries in the Western Hemisphere."

The possible indictment follows renewed calls from South Florida lawmakers and Cuban exile groups demanding that Raul Castro be prosecuted for the attack after the 30th anniversary of the shootdown passed earlier this year. Republican lawmakers including Rep. Carlos Gimenez, R-Fla., Rep. Maria Elvira Salazar, R-Fla., and Rep. Mario Diaz-Balart, R-Fla., have publicly urged the Trump administration to pursue criminal charges.