At least 11 people are dead and dozens were rescued after a boat capsized Thursday off the coast of Puerto Rico, officials said.

The vessel, which was “suspected of taking part in an illegal voyage,” overturned shortly before noon local time Thursday, the U.S. Coast Guard said in a statement.

A U.S. Customs and Border Protection aircraft spotted the capsized vessel and people in the water about 11 miles north of Desecheo Island, a small, uninhabited wildlife refuge off Puerto Rico’s western coast.

The Coast Guard and Customs and Border Protection responded along with the marine units of the Puerto Rico Police Department. The Coast Guard launched multiple helicopters from Air Station Borinquen and diverted the Coast Guard Cutter Joseph Tezanos as part of the rescue operation.

There were 31 survivors, according to the Coast Guard. None of the passengers appeared to be wearing life jackets, it said.

The total number of passengers who were on the boat before it capsized is unclear.

The vessel was transporting non-U.S. citizens, the Coast Guard Office in Puerto Rico said.

Desecheo Island is located in the Mona Passage, a potentially dangerous strait that separates the islands of Hispaniola and Puerto Rico.