The U.S. and Colombia continue to join forces in the fight against narcotics trafficking, but there are incidences that derail those efforts and claim lives.

The twin-engine turboprop plane that crashed in northern Colombia on a U.S. counter-drug mission, killing three American contractors and a Panamanian aboard, had been tracking a suspected smuggling vessel over the western Caribbean when it lost radio contact, according to the U.S. military, the Associated Press reports.

The two other Americans aboard the Dash 8 were seriously injured in Saturday's pre-dawn crash, the U.S. Southern Command said.

"We express our sympathies to the families of the deceased," General John Kelly, commander of U.S. Southern Command, said in the statement. "The U.S., Panama, Colombia, and all of our partners in Operation Martillo remain committed to our longstanding cooperative relationship for counter-narcotics operations."

Under contract with the U.S. Air Force, the plane was a "prospector" aircraft, equipped with surveillance equipment and employed to track speedboats and other vessels that smuggle cocaine north from Colombia, said Jody Draves, spokeswoman for the U.S.-sponsored multinational task force in Key West, Fla., known as JIATF-South that runs interdiction in the region.

According to the AP, Draves said the plane was operating in coordination with a Colombian surface vessel and had flown out of Panama. It was near that country's border that it crashed, near the city of Capurgana.

Colombian soldiers came to the aid of the two injured Americans, who were brought to a hospital near the capital of Bogota.

The names of the three diseased Americans have not been released, pending notification of next-of-kin; however, Panama's National Air Service identified the deceased Panamanian guardsman as Lt. Lloyd Nunez.

Is there suspicion that the plane was taken down by the FARC (Revolutionary Armed Forces of Colombia)?

According to Gen. Nicasio de Jesus Martinez, commander of the Colombian army's 4th Brigade whose troops traveled to the accident scene, ruled out the possibility that the plane was shot down by rebels active in Colombia.

"There was no aggression, no impact," said Martinez, adding that it was too soon to know if the crash was caused by mechanical failure, human error or the weather. Southcom also said there was no indication the plane was shot down.

The plane crashed in a dangerous mountainous jungle where the FARC thrive and operate there along with drug traffickers.

Washington also gives aid to Panama, a transit country between Colombia and Mexico, home to the major cartels that supply cocaine to the U.S. market, Reuters reports. Last week, a U.S. pilot was killed and his co-pilot injured while carrying out the government-contracted task of spraying coca plants used to produce cocaine in the south of Colombia.

Operation Martillo (Hammer) is part of the $165 million, U.S.-led regional security initiative that focuses on the seas off Central America, key shipping routes for 90 percent of the cocaine headed to the U.S. Fourteen countries participate: Belize, Canada, Colombia, Costa Rica, El Salvador, France, Guatemala, Honduras, the Netherlands, Nicaragua, Panama, Spain, United Kingdom and the United States. Chile has also contributed to the operation.

This incident comes at a time when Colombia's current President Juan Manuel Santos is feuding with his predecessor and former boss, Álvaro Uribe, which may ultimately affect peace talks with the FARC in the future.

"The relentless attacks from Uribe, who was the closest U.S. ally in Latin America during his eight-year rule, is raising eyebrows among policymakers in the Obama administration and in the U.S. Congress," The Washington Post reports. "Both Uribe and Santos were central to the war on drugs in the Andes, a battle largely funded with billions of dollars in U.S. military aid, and analysts say the effort to undermine the Santos presidency is seen as unseemly."

"Uribe, 61, whose father was killed by FARC guerrillas, contends that Santos betrayed his administration's legacy by repairing relations with Venezuela's leftist government, which assisted the rebels, and then embarking on negotiations with the group's commanders. He does not mention that when Santos was his defense minister, the Colombian security services delivered their most decisive blows against FARC.

Uribe took to Twitter to continue bashing the President, where he tweeted to his 2.3 million followers that Santos has "converted terrorists who were in the process of being defeated and isolated into political actors."