Aerial search crews working in adverse weather conditions late today located wreckage that authorities believe is the U.S. military reconnaissance plane that disappeared Friday during a routine anti-drug mission over a mountainous jungle region of southern Colombia controlled by leftist guerrillas.

"There's reason to believe the aircraft is down. The wreckage site is under observation and efforts to get to that site are underway," Gen. Barry R. McCaffrey, the U.S. national drug control policy director, said at a news conference in Miami before leaving for Bogota on a previously scheduled trip.

One military source said that authorities had picked up an electronic signal in guerrilla-controlled Putumayo province near the border with Ecuador that may have been received from tracking equipment aboard the missing De Havilland RC-7 plane, but low cloud cover had prevented searchers from confirming that it was the aircraft.

McCaffrey said it was not known when investigators would get to the plane, which carried five U.S. Army soldiers and two Colombian air force officers.

"That will unfold in the coming 24 hours," he said. As night fell, officials said that the earliest any confirmation could be made would be Monday morning.

U.S. military officials said the missing Americans are two captains, a warrant officer and two enlisted men from Fort Bliss, Tex.

The U.S. Southern Command in Miami, which is responsible for military operations in Latin America and the Caribbean, said search teams had made a "visual sighting" of wreckage late this afternoon. But because of the weather, officials said, they had not been able to get close enough to the site to identify the plane, most likely by its tail number.

Frustrated officials said that a number of the 25 aircraft used on Saturday to scour the province were waiting for the weather to clear.

"Unfortunately, it turns this into a waiting game, which nobody wants," said Navy Lt. Jane Campbell, a spokeswoman for the Southern Command.

Those search aircraft that did take to the skies were delayed by thick fog this morning that shrouded the mountainous terrain. Once airborne, some crews could only survey territory below a 7,000-foot cloud ceiling in an area where mountain peaks tower as high as 12,000 feet.

Reports about the plane have continued to trickle in to investigators from towns in and around the search area, where residents claim to have either seen the aircraft flying at a low altitude or heard an explosion at around the time that it lost communication with military authorities.

The plane was reportedly looking for cocaine-producing laboratories as part of a stepped-up U.S. effort to stem the flow of drugs from this South American country. The RC-7 usually flies such missions at altitudes between 5,000 and 25,000 feet.

Colombia supplies an estimated 80 percent of the world's cocaine and more than two-thirds of the heroin consumed in the United States.

Two-thirds of the coca grown in Colombia comes from the Putumayo and Caqueta regions, which are strongholds of the Revolutionary Armed Forces of Colombia, the oldest and most powerful Marxist insurgency in Latin America. Coca is the raw material of cocaine.