A U.S. State Department plane used to fumigate drug crops crashed Monday and its American pilot was killed, the U.S. Embassy said.

It was not immediately clear if the crash was caused by an accident or if the T-65 Air Tractor plane had been shot down, the embassy said.

The American, whose name was not released pending notification of relatives, was the fourth to die in three crashes of U.S. government planes in Colombia this year.

Three other Americans were killed when their single-engine Cessna plane crashed and burned in southern Colombia on March 25 while searching for three other Americans who were captured by leftist rebels after their plane went down on Feb. 4. The rebels executed a fourth American and a Colombian soldier while taking the others as hostages, Colombian officials have said.

Monday's crash occurred in southwest Colombia's Narino state, where there are large plantations of coca, the main ingredient of cocaine. Leftist rebels, who along with their paramilitary rivals oversee cocaine production in Colombia, have fired at the spray planes in the past.

One U.S. official, speaking on condition of anonymity, acknowledged that it was unusual that three planes would go down in such a relatively short span of time.

But U.S. officials have often said — both in comments to journalists in Colombia and in testimony to the U.S. Congress — that the anti-drug missions in Colombia are not risk free.

The crop-dusters, flown by State Department contractors, fly close to the ground as they fumigate the coca bushes, and often receive bullet impacts. U.S. special forces troops have trained Colombian troops who conduct ground sweeps to try to protect the spray planes, but it is impossible to remove every threat.

The U.S. Embassy had no further details on the latest crash. DynCorp, of Reston, Va., was contracted to conduct the fumigation flights. DynCorp officials could not immediately be reached for comment.

Meanwhile, hundreds of Colombian troops, assisted by U.S. intelligence and operations planners, continue to comb the mountains and jungles in search of the three American captives. The Revolutionary Armed Forces of Colombia, or FARC, have said they want to trade the three men for rebels being held in government prisons.

Colombia produces most of the cocaine in the world. Hardline President Alvaro Uribe has welcomed U.S. aid, which aims to dramatically cut the amount of coca under cultivation. Profits from the country's cocaine industry, controlled largely by the FARC and a handful of right-wing paramilitary groups, fuel the country's civil war, now in its 38th year.