The U.S. Coast Guard said the containers slid into the Atlantic about 18 miles off Key Biscayne Monday afternoon, the South Florida Sun Sentinel reported. The Atlantic Trader returned to Port Everglades, Fla., with about half its containers still onboard.
The barge was offloaded Tuesday.
The barge has been making trips twice a month to the U.S. base at Guantanamo for years. It is the base's primary supply line, carrying most of what's needed for the detention center, the base and the men and women stationed there.
Coast Guard spokeswoman Marilyn Fajardo told The Miami Herald the cargo included some potentially hazardous or polluting materials, including "combustible liquids, possibly diesel fuel."
Fajardo said the Coast Guard was working with other agencies to investigate possible water pollution.
A tugboat escorting the barge first notified the Coast Guard of the spill.