Colombia's director of domestic intelligence resigned Thursday after her agency was caught spying on a prominent political opponent of President Alvaro Uribe.

Maria del Pilar Hurtado called her resignation after 14 months as head of the Administrative Department of Security, or DAS, "an act of dignity" in a statement she read to reporters. She did not take questions.

Hurtado said neither she nor Uribe ordered the surveillance of Sen. Gustavo Petro, a member of the leftist Polo Democratico party and a key figure in efforts to uncover ties between political allies of the president and far-right death squads.

Hurtado's resignation comes two days after Petro said he anonymously received two incriminating memos signed by her intelligence chief that ordered regional DAS offices to investigate the senator and his party. Hurtado fired the intelligence official on Wednesday.

The memos were dated August and September. One ordered that information be gathered on Petro's "contacts with people who offer to testify against the government."

The resignation is only the latest scandal to taint the DAS, which reports directly to the president and is responsible for both domestic intelligence and keeping track of foreigners in the country.

In 2005, a previous agency director and close Uribe ally, Jorge Noguera, was forced to resign over allegations he colluded with the illegal far-right militias known in Colombia as paramilitaries.

Documents found in a paramilitary lieutenant's computer say Noguera provided the militias with names of union and human rights activists, some of whom were later murdered.

First formed in the 1980s by ranchers and drug traffickers to counter leftist rebels, the militias evolved into mafias that killed thousands and stole millions of acres.

Petro began to publicly denounce ties between Uribe-allied politicians and the militias in 2005, with witnesses approaching him who did not trust the chief prosecutor's office to protect them.

Subsequently, the Supreme Court ordered the arrest of more than 30 members of Congress for allegedly benefiting politically or financially from ties with the so-called paramilitaries.

Noguera, meanwhile, was arrested in February 2007 and charged with criminal conspiracy but released in June by a court that ruled prosecutors made a procedural error.