It's never been a secret that the United States has heavily funded and influenced the Colombian government's battle against FARC, which the U.S. believes to be a terrorist organization. What has been secret however, is just how far the CIA has gone in Colombia, which includes the targeted killings of FARC leaders.
In a sweeping investigative piece published by the Washington Post today, journalist Dana Priest explains that in 2006, the Unites States Air Force proposed the use of "smart bombs" as a way to kill the leadership of the Marxist organization that has been fighting the Colombian government for decades. The Bush White House concluded that the killing would be considered legal, as FARC posed serious threat to the Colombian government (who also happens to be the United States's strongest ally in the region).
The United States then went on to set a far-reaching precedent when it decided to aid Colombia in its killing of the second-in-command of FARC, who was living in Ecuador at the time. The U.S. determined then that if a country was harboring someone it perceived as a terrorist, and making no attempts to arrest them, then the U.S. has the go-ahead to assassinate them wherever they may be. In 2008, it was FARC leader Raul Reyes in Ecuador. In 2011, it was Osama Bin Laden in Pakistan.
In 2010, the CIA handed off control of the smart-bomb project entirely to the Colombians. Since then, FARC has been decimated by attacks from the Colombian government, attacks made possible by United States technology, and working mostly off of United States intelligence reports.