Updated

Authorities were trying to determine the origin of a fuel spill that could threaten coral reefs off Puerto Rico, the government said Thursday.

Javier Velez Arocho, secretary of the Department of Natural and Environmental Resources, said the slick is believed to be bunker oil, which is used to power older-model ships.

The size of the spill was not immediately clear, although Velez said after touring the area by helicopter that it covered about 3 miles.

Velez said it probably occurred in the Caribbean and then drifted toward Puerto Rico's southern coast, where it poses a risk to already-threatened reefs.

Investigators inspected three ships anchored in Guayanilla Bay and questioned their owners to try to discover the origin of the spill.

The U.S. Coast Guard is also investigating, Velez said.