A secret Obama administration memo disclosed Monday outlines the legal justification for the government's drone-targeted killing program, a lethal strategy that authorizes the killing of innocents as collateral damage.

The memo (PDF), released by a US federal appeals court under a Freedom of Information Act request, describes the government's legal underpinnings for its so-called overseas targeted-killing program where drones from afar shoot missiles at buildings, cars, and people. It began under the George W. Bush administration but was broadened under Obama and now includes the killing of Americans.

The Obama administration fought for years to keep the Justice Department Office of Legal Counsel memo from becoming public. The document says that lethal force is authorized under international war rules and the US war on terror. Rights groups, however, decried the 41-page document, saying that it amounted to a legal blueprint for other nations to follow.

"While today the US, the UK, and Israel are the only countries known to have used killer drones, experts say that within 10 years virtually every country on earth will be able to build or acquire drones capable of firing missiles. The United States loosening and redefining international rules governing the use of force and war is ultimately not going to make anyone any safer," Pardiss Kebriaei, a senior attorney with the Center for Constitutional Rights, said in a statement.

The memo in part concerns the killing of Anwar al-Awlaki, a New Mexico native and radical cleric who the authorities said was an al-Qaida recruiter along the Arabian Peninsula and was associated with the September 11 hijackers. He was killed by an American drone strike more than two years ago in Yemen. Also killed in 2011 Yemen drone bombings were the cleric's 16-year-old son, Abdulrahman, and Samir Khan, who was editor of the English-language al-Qaida publication Inspire.

Parts of the memo mirrored the administration's successful defense of a lawsuit brought by relatives of those three in which the government said that the killings were justifiable acts of war. But the internal memo unveiled Monday says that the Department of Defense and other agencies like the Central Intelligence Agency may carry out extrajudicial killings of its enemies—even if they are Americans and regardless of whether or not innocent lives are taken in the process.

"DoD has represented to us that it would make every effort to minimize civilian casualties and that the officer who launches the ordnance would be required to abort a strike if he or she concludes that civilian casualties would be disproportionate or that such a strike will in any other respect violate the laws of war," according to the "memorandum for the attorney general."

But that didn't sit well with Kebriaei. In a telephone interview, the Center For Constitutional Rights attorney said that "if you accept the idea of a global war and you can follow a target wherever he goes, there is a significant risk of harm to civilians in the area precisely because the laws of war do allow some collateral harm. It's basically a huge risk of harm to ordinary people and civilians if you accept this premise which can be invoked by other countries."

The 41-page document was written by David Barron, who the Senate confirmed to a seat on a federal appeals court last month.

Jameel Jaffer, the deputy legal director of the American Civil Liberties Union that sued for the document's release, speculated in a Monday statement that "The drone program has been responsible for the deaths of thousands of people, including countless innocent bystanders."

The government has refused to divulge how many people it has killed with drones.

The memo also said the cleric's killing was justified by Congress, which in the immediate aftermath of the 2001 World Trade Center terror attacks authorized retaliation against al-Qaida. "The contemplated DoD operation, therefore, would be carried out against someone who is within the core of individuals against whom Congress has authorized the use of necessary and appropriate force," the memo said.

It does not matter whether the strikes target Americans, either. US citizenship, the memo said, does not impose "constitutional limitations that would preclude the contemplated lethal action under the facts represented to us by DoD, the CIA, and the Intelligence Community."