(Updates with Chertoff quotes, police killing)
SALINA CRUZ, Mexico, July 18 (Reuters) - The Mexican military, working with information from U.S. intelligence services, found nearly six tonnes of cocaine in a makeshift submarine seized this week off the Pacific coast.
The 32-foot (10-metre)-long, green fiberglass craft was designed to travel just beneath the water, leaving almost no wake.
It was one of Mexico's largest maritime drug seizures and the first time the country has seen drug smugglers using a submarine, the navy said.
On Friday the army laid out more than 200 packages of drugs, tightly wrapped in black plastic, on the dock where the vessel was hauled in.
U.S. Homeland Security Secretary Michael Chertoff, in Mexico to discuss an aid package of more than $400 million to fight drug gangs, said the United States had a minor role in the operation.
"We shared information with the Mexican navy, but the Mexican navy acted alone. It was actually their seizure, their marines, their helicopters and naval vessels that captured the submarine," Chertoff told reporters in Mexico City.
Four Colombians aboard the submarine said they had navigated up the Pacific coast from Colombia, the navy said.
Colombian officials told Reuters last month that diesel-powered drug submarines travel up to two weeks to reach Central America and Mexico. The drugs are then hauled overland into the United States.
Mexican special forces raided the submarine on Wednesday after they spotted it from the air by helicopter. They detained the crew and brought them and the vessel back to the Pacific port of Salina Cruz in Oaxaca state.
Chertoff said the troops swooped down onto the craft using ropes from a hovering helicopter before the crew had time to sink the ship.
"This is going to force us to increase surveillance," Vice Adm. Jose Maria Ortegon told reporters in Salina Cruz.
The government has made several huge drug seizures by deploying thousands of troops to trafficking hot spots after President Felipe Calderon took office in December 2006.
But drug trade specialists say troops and police are failing to tackle drug gangs' financial networks and go after money launderers, which they say would do more to weaken the cartels.
Some 1,700 people have been killed in drug gang violence in Mexico so far this year, and Calderon's frontal assault has failed to stop attacks on police and soldiers.
Drug hitmen shot and killed a policeman in his office in the northern border city of Ciudad Juarez on Friday, the first time gunmen have penetrated a police building to murder an official in the city, police said. (Additional reporting by Mica Rosenberg in Mexico City, Lizbeth Diaz in Tijuana and Ignacio Alvarado in Ciudad Juarez)
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