In the months before two U.S. drug surveillance planes crashed in Colombia, government contract workers repeatedly warned their employers and the U.S. military about the dangers of using single-engine Cessna 208 planes for their risky missions.

In the last six weeks, four Americans and a Colombian have died and three Americans have been kidnapped by leftist rebels in a pair of Cessna crashes during antinarcotics missions flown by Defense Department civilian contractors.

Taken together, the two crashes and the unheeded warnings raise questions about the safety of the antidrug program here, where the U.S. government relies on private companies to do much of the work.

"Every time I had a night mission, I'd come back and almost kiss the ground because I knew we had survived another one," said Douglas C. Cockes, one of two former pilots who warned their employers of significant safety concerns in a series of letters written over the last three months.

"We could see the red flags waving in the wind and we merely wanted people to know, hey, before impact, let's think about this," said Cockes, who worked until January for California Microwave Systems, a Northrop Grumman Corp. subsidiary with a Pentagon contract to monitor the Colombian jungle for coca fields and processing plants. Cockes said he resigned because of conflicts with management over safety and personnel issues.

The U.S. has invested nearly $2 billion in the last several years to combat drugs in Colombia, which produces 90% of the cocaine sold on U.S. streets.

The government has hired private contractors to perform the bulk of the antidrug mission. Private companies do everything from spraying fields of coca, the base of cocaine, to building new wells for poor farmers as an incentive to eschew the lucrative drug trade.

U.S. officials have long defended the contractors as the most efficient use of taxpayer funds. Privately, some also admit that the contractors allow them to deflect attention from growing U.S. involvement in Colombia's messy, four-decade internal conflict.

In a briefing for reporters this month, after the crash of the first Cessna 208 but before the most recent accident Tuesday night, U.S. officials strongly defended the use of single-engine airplanes.

The issue is not the quality of the Cessna, but rather whether it is powerful enough for this type of operations.

"The single-engine business in my view is an argument that is not worth having," the U.S. official said. "We've been flying those planes in Colombia for over three years. We fly other single-engine planes in the United States military. An F-16 fighter jet is a single-engine aircraft."

But Rep. Janice D. Schakowsky (D-Ill.), who has long been critical of the use of contractors, said the crashes have convinced her to reintroduce a bill this Congress to ban private military contractors from operating in the Andean region.

"The recent tragic death of American private military contractors underscores the dangers of U.S. involvement in Colombia," Schakowsky said Wednesday.

The most recent crash came Tuesday night, when U.S. contractors flying a Cessna 208 hit a mountain while searching for three California Microwave Systems crew members who were kidnapped by leftist rebels after their Cessna 208 crashed in the same area in February.

Two crew members in the February crash, Thomas John Janis, 56, a decorated former U.S. Army officer working for California Microwave Systems, and Colombian intelligence Sgt. Luis Alcides Cruz, were killed on the scene by rebels from the Revolutionary Armed Forces of Colombia, or FARC, according to Colombia and U.S. authorities. The plane was apparently on a mission to map drug fields for future spraying operations.

Since then, the U.S. Embassy and the Colombians have mounted a massive search and rescue effort to find the men. The FARC have announced the kidnapped men are being held as hostages to exchange for jailed guerrillas.

The Cessna that crashed Tuesday was part of that operation when it took off about 7 p.m. from Larandia, a military base about 220 miles southwest of Bogota. The fuel-laden plane crashed into a mountain and burst into flames.

Colombian television showed a scene of the small plane's pieces strewn over a mountainside and local media reported it had crashed into a communications tower on a hillside near the town of El Paujil five minutes after reporting mechanical failure.

U.S. Embassy officials confirmed Wednesday that all three crew members in the search and rescue plane had been killed, but they declined to identify them or name the company they worked for.

Jack M. Martin Jr., a spokesman for Northrop Grumman's electronics systems sector, said the plane was provided and operated by a subcontractor to California Microwave Systems.

Martin said the company was "deeply saddened" by the crash, but had no further comment because of the ongoing U.S. investigation.

U.S. and Colombian officials have said they believe that engine troubles were the cause for both plane crashes.

Cockes, a former U.S. Customs pilot with nearly 30 years' experience in drug interdiction missions, said he and fellow pilots orally warned California Microwave Systems in May 2002 that the missions being undertaken by the Cessna 208 put them at greater risk.

That warning was backed up by letters to both the company and parent firm Northrop Grumman in November and December, Cockes said, providing copies to The Times.

The pilots emphasized that the plane had only a single engine and that it had difficulty climbing above 15,000 feet, particularly in the icy conditions often encountered when flying in the Andes.

If the engine failed, they wrote in one letter, the Cessna "could not reach any suitable landing area" in most of the terrain it covered. The planes frequently fly over remote areas where Colombia's soaring mountains, site of most heroin-base crops, meet the vast, unpopulated eastern plains, site of most coca crops.

The Cessna 208, which crashed in February, was flying at 17,000 feet when it had engine trouble, a U.S. Embassy official said.

In their letters, the pilots recommended that the company replace the Cessna with Beech King Air 300 twin-engine turboprops that can fly at higher altitudes and more effectively at night and in icing conditions. But the company declined to use the more expensive twin-engine planes, Cockes said.

"The continued use of this [Cessna 208 plane] invites a catastrophic impact in mountainous terrain if there is an engine failure," the pilots wrote in a Nov. 14, 2002, letter.

Cockes accused the company of ignoring the warnings.

"When the guy pulled the trigger and blew off the back of the head of Tom Janis, they were part of the trigger-pull on that, the company was, absolutely," he said in an interview.

Recently, the pilots sent copies of both letters to officials at Southern Command, the Miami-based task force that oversees U.S. military operations in Latin America, Cockes said.

Raul Duany, a spokesman for Southern Command, acknowledged that the letters were discussed by command officials earlier this month.

He said the command suspended operations to address safety concerns after the February crash.

Duany said he did not want to speculate on whether the two crashes were related, but said the command takes the safety of its personnel seriously.

"We are investigating the circumstances surrounding this unfortunate event," he said.

Louis Ponticelli, the father of one of the men killed in the crash, former U.S. army soldier Ralph Ponticelli, 42, said his son rarely spoke about the company he worked for, but that he maintained an office at the U.S. Embassy in Bogota.

"He was a responsible individual," said Ponticelli, who said his son left a 20-year-old daughter. "He did what he could ... for his country."