A freighter grounded on a reef near Puerto Rico on Thursday, and dozens of people aboard swam to a nearby island. The U.S. Coast Guard said it is investigating.

Officials have accounted for about 60 people who evacuated the vessel and were searching for any who might have remained aboard, said Coast Guard spokesman Ricardo Castrodad.

"This is extremely unusual," he said.

Castrodad said he did not know where the 185-foot (56-meter) freighter had come from, where it was going or whether it had any cargo aboard. It was registered in Honduras under the name "Jireh."

Daniel Kercado, secretary of Puerto Rico's Natural Resources Department, said in a statement earlier that authorities were trying to rescue about 100 Haitian migrants stranded on the island of Mona after their boat hit a reef. He said about 80 Haitians swam ashore and that another 20 remained on the boat.

He said the migrants told authorities they were headed to the French island of St. Martin.

Castrodad, however, said federal agents were still trying to determine the nationality of those aboard the freighter.

Natural Resources Department spokeswoman Jailyn Perez said it was one of the largest groups of migrants ever found stranded. She also said it is far more common for Dominican and Cuba migrants to be found in the area.