Updated

A report of three rifles missing from military barracks triggered the investigation into a former Salvadoran army officer who pleaded guilty this week in the United States to federal terrorism charges, officials in this Central American country said Thursday.

Hector Antonio Martinez-Guillen, 32, acknowledged Wednesday in a U.S. District Court in Virginia that he sold automatic rifles, ammunition and plastic explosives to a member of the Revolutionary Armed Forces of Colombia, or FARC. The U.S. government has designated the leftist rebel group as a terrorist organization.

Brig. Gen. Francisco Ramon Salinas, El Salvador's chief of defense, said the U.S. officials began investigating Martinez after the U.S. Drug Enforcement Administration received a report on the case from El Salvador's Armed Forces in 2009.

The three M-16 rifles went missing at the Salvador special forces barracks where Martinez-Guillen was a captain.

"We gave DEA the corresponding report, we gave the reports to police. We joined efforts" for his detention, Salinas said.

Salinas said Martinez-Guillen resigned in May 2010, but the investigation continued.

The DEA, which has an office in El Salvador, carried out a nearly yearlong sting operation in which an undercover agent met with Martinez-Guillen in a hotel in San Salvador in July 2010.

The operative told the Martinez-Guillen that the rebel group would kill American troops and military consultants in Colombia if given a chance. The ex-captain agreed to provide military rifles to a person he believed was working with FARC anyway.

The sting concluded later that year, authorities said, when Martinez-Guillen flew to Washington Dulles International Airport in northern Virginia to sell more than $1 million of cocaine in what he thought was a FARC drug deal. He was arrested and has remained in custody.

Prosecutors agreed as part of a plea bargain they would seek no more than 45 years at his sentencing, scheduled for July 29.

U.S. Attorney for the Eastern District of Virginia Neil MacBride said the investigation continues.