National Security Council spokeswoman Caitlin Hayden says the United States is deeply concerned by Rafael Caro Quintero's release from prison Friday.
Defense lawyers believe a second drug kingpin convicted in the death of U.S. Drug Enforcement Administration agent Enrique "Kiki" Camarena may also be freed.
An appeals court overturned Caro Quintero's 40-year sentence for kidnapping, torture and murder on procedural grounds.
Hayden said in a statement Sunday that the United States is working with Mexican authorities to see that those responsible for Camarena's murder face justice in the U.S.
Copyright 2013 The Associated Press.