Isaac strengthened on Wednesday and looked set to become a hurricane as it churned through the Caribbean on a path that could bring it to Florida during next week's Republican convention. The tropical storm, packing gusts of up to 45 mph, was expected to intensify and "could become a hurricane by Thursday," the National Hurricane Center said. (NASA/AFP/GETTY IMAGES)

A tropical storm gathering strength in the Caribbean forced the U.S. military on Wednesday to postpone the latest hearings for the five detainees charged in the Sept. 11, 2001, attacks, as officials evacuated staff members and others from Guantanamo Bay.

Tropical Storm Isaac is forecast to make landfall Saturday morning near the U.S. naval base on the southeastern tip of Cuba, according to the National Weather Service. It is expected to become a hurricane by Thursday.

On Wednesday, a military spokesman at the Guantanamo detention center said most of the detainees are housed in concrete structures that can withstand the effects of hurricane-force winds; those who are not will be transferred to secure structures. Officials said they are also preparing to evacuate nonessential personnel, representatives of human rights groups and reporters from the island.

Residents of the base, meanwhile, were told to expect destructive winds and were advised to secure loose objects in their yards that could “become projectiles.”

The last major storm to threaten Naval Station Guantanamo Bay was Tropical Storm Tomas in November 2010. It passed just east of Cuba but brought heavy rain and 60 mph winds, causing substantial flooding in the region.

Across the Caribbean, Tropical Storm Isaac has forced evacuations and the closures of schools and government offices. It has also threatened to upend plans for the Republican National Convention in Tampa, which begins Monday, with some weather models suggesting a hurricane path that could run through Florida early next week.

The hearings in the Sept. 11 case had already been postponed by one day, after a deadly train derailment in Maryland interrupted the government’s computer network on the island. No new date for the hearings has been set.

The judge in the proceedings, Army Col. James Pohl, was expected to hear a series of motions in the case against Khalid Sheik Mohammed, the self-described mastermind of the 2001 attacks, and his four alleged co-conspirators.

One of the motions was filed jointly by the American Civil Liberties Union and 14 media organizations, questioning the rules of classification concerning what detainees tell their lawyers and what they, in turn, can argue in the public domain.

The five men are charged with murder in violation of the law of war, hijacking and terrorism, among other charges.

After their capture, they were held at secret CIA prisons overseas before being transferred in September 2006 to Guantanamo, where they are held at a small, high-security facility known as Camp 7.

The defendants last appeared in court in May. They refused to speak during those proceedings, instead sitting silently while the 87-page charge sheet, including the names of all 2,976 victims of the Sept. 11 attacks, was read.