Three Americans who vanished after their U.S. plane crashed during an intelligence-gathering mission last week are believed held by a Colombian rebel faction known for daring kidnapping operations, officials said.

Colombian troops made no progress Tuesday in their search for the missing Americans, whose plane crashed in guerrilla territory on Thursday, officials said.

The Teofilo Forero unit of the Revolutionary Armed Forces of Colombia, or FARC, are apparently holding the Americans, Colombian military officials said.

The Teofilo Forero is believed to be under the direct orders of the FARC's top military commander, German Briceno, and has conducted several daring operations. In April, they kidnapped legislators in Colombian city of Cali while posing as government bomb-squad members. On Feb. 20, 2002, they hijacked a domestic airliner, forced it to land on a rural road and kidnapped a Colombian senator who was aboard.

The Americans were on an intelligence mission when their U.S. government single-engine Cessna crashed Thursday in the Colombian jungle. A fourth American and a Colombian army sergeant aboard the plane were found shot to death at the site.

"A joint effort is underway, of our armed forces with the support of the Embassy of the United States, to recover these three people," Foreign Minister Carolina Barco told reporters Tuesday.

The U.S. government has not identified the people on the flight or disclosed their mission. Several U.S. agencies and government contractors are in Colombia. They operate radar stations that track drug-smuggling flights, fumigate drug crops with airplanes and assist Colombian security forces in other anti-drug operations.

A U.S. military official said Tuesday that the United States is helping search for the trio in the jungles and mountains of Caqueta. The remains of the fourth American killed at the scene were sent to the United States on Sunday, officials said.

Colombia's 38-year civil war pits the 17,000-strong FARC and a smaller rebel group against the government and right-wing paramilitary fighters. About 3,500 people, mainly civilians, die in the fighting each year.