HUATULCO, Mexico – Mexico's navy seized a homemade submarine carrying a drug shipment off the Pacific coast on Wednesday and arrested its four-man crew.

Similar vessels carrying cocaine have been discovered off Colombia and Central America, but navy spokesman Capt. Benjamin Mar said the seizure is a first for Mexico.

The 30-foot (10-meter) makeshift submarine was detected heading north about 200 miles (322 kilometers) off the southern state of Oaxaca, Mar said.

Green-topped and shaped like an arrowhead, the vessel was intercepted when it surfaced hours later. The crew was taken into custody without resistance.

The suspects were flown by helicopter to the city of Huatulco, where they told AP Television News they left the Colombian coastal town of Buenaventura a week ago. They said drug traffickers forced them to make the journey by threatening to harm their families.

The navy said in a statement that the sub was apparently packed with cocaine, but authorities were still determining how much was on board.

Colombia's drug cartels have been known to use home-built submarines to smuggle large amounts of cocaine past U.S. and Colombian patrol boats to Central America en route to the United States.

Colombian authorities have discovered at least nine such vessels over the past three years. Last August, U.S. forces intercepted a submarine-like vessel carrying tons of cocaine off the coast of Guatemala.