Tunja, Colombia ? A former drug kingpin was freed late Thursday after serving only half his sentence, despite U.S. efforts to find evidence to support further charges ” and possibly his extradition to the United States.

Gilberto Rodriguez Orejuela, who with his brother Miguel once controlled the Cali drug cartel, an empire that moved multiton shipments of cocaine across the globe, walked out of prison shortly after 10 p.m.

Rodriguez, who was arrested in 1995 and sentenced to prison until 2010, last week was ordered released by Judge Pedro Suarez, for good behavior and participation in a prison work study program. As the government investigated Suarez to see if the convicted drug trafficker might have bribed him ” a charge that Suarez has denied ” another judge upheld the decision Thursday.

Tensions mounted Thursday amid expectations that Rodriguez might be freed. Dozens of police and soldiers surrounded the prison where he was held, outside the town of Tunja, 60 miles northeast of Bogota, to prevent any violence.

With the clock ticking, U.S. officials tried in vain to stop his release.

“Some documents have arrived from the United States that officials are evaluating, and that could stop the release,” President Alvaro Uribe’s spokesman, Ricardo Galan, said.

Details on what information was being provided were not immediately available, but U.S. drug agents have been trying to link Rodriguez and his jailed brother Miguel to international crimes committed after 1997, when Colombia’s constitution was revised to allow the extradition of its citizens.

Suarez’s decision last week that the Rodriguez brothers should be freed shocked the nation, and prompted the president to intervene. But Judge Luz Amanda Moncada ruled Thursday that Suarez’s order on Gilberto Rodriguez should stand. She also ordered an investigation of the government for allegedly interfering in the judicial process.

Interior and Justice Minister Fernando Londono called the ruling a “terrible blow.”

A Colombian court has ordered the release from prison of Gilberto Rodriguez Orejuela, center, one of the country's former top drug lords. But investigators are scrambling to find evidence to support further charges and, possibly, his extradition to the United States.