The American Civil Liberties Union has objected to the killing of the U.S.-born Muslim cleric Anwar al-Awlaki in Yemen by U.S. forces.

Awlaki was a U.S. citizen, and the ACLU said President Obama does not have the authority to kill an American without due process of law. The White House confirmed the cleric was killed by a U.S. drone attack.

“The targeted killing program violates both U.S. and international law,” ACLU Deputy Legal Director Jameel Jaffer said. “As weve seen today, this is a program under which American citizens far from any battlefield can be executed by their own government without judicial process, and on the basis of standards and evidence that are kept secret not just from the public but from the courts.”

The ACLU said the government only has the authority to kill Americans when a threat is imminent.

“It is a mistake to invest the president — any president — with the unreviewable power to kill any American whom he deems to present a threat to the country,” Jaffer said.

Ben Wizner, litigation director of the ACLU’s National Security Project, added:

“If the Constitution means anything, it surely means that the president does not have unreviewable authority to summarily execute any American whom he concludes is an enemy of the state.”

Obama’s actions also garnered criticism from GOP presidential candidate Rep. Ron Paul (Texas).