Former Argentine army officer Alejandro Lawless, jailed for crimes against humanity under military rule, escaped on Tuesday while in transit to a Buenos Aires court.
Lawless, 66, fled at midday from a police van outside a court building where he was being taken along with other prisoners.
The security guards in charge said he took advantage of their distraction while they led the others from the van to the courthouse to make his escape.
Lawless was sentenced in 2010 for his involvement in kidnapping, torturing and killing political opponents of the government between 1976 and 1983.
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Prosecutors said "he formed part of a criminal plan to kidnap, torture, kill and disappear people using the structures of the armed forces."
He is the third convicted military officer to escape from prison this year, following two ex-army officers, Jorge Olivera and Gustavo de Marchi, who broke out of a military hospital in July. Both are still on the run.
Human rights leader Estela de Carlotto denounced what she called the "complicity" in Lawless's jailbreak.
"It is negligence or, to think the worst, complicity. This is serious, it will have to be investigated and those responsible sanctioned," Carlotto told reporters on Wednesday.
An estimated 30,000 people were killed during Argentina's seven-year military rule known as the Dirty War.