A U.S. Customs Service officer who became suspicious Wednesday when a top-heavy camper lumbered onto a Rio Grande bridge began an inspection that uncovered 1,000 pounds of cocaine worth an estimated $187 million, an official said.

"It had a false roof. That is, it had an additional roof built on top of the existing roof, and this empty space at the top of the Winnebago was filled entirely with packaged cocaine," said Charles Conroy, a Customs spokesman in Houston.Two men and a woman were arrested, he said. They were being questioned by federal authorities.

Conroy said inspectors had found about 1,000 pounds of cocaine in 2-kilogram bricks and were tearing apart panels looking for more.

Conroy said the cocaine had an estimated retail value of $187 million.