"Defendants must be trusted and expected to act in accordance with the U.S. Constitution when they intentionally target a U.S. citizen abroad at the direction of the president and with the concurrence of Congress," Collyer said. "They cannot be held personally responsible in monetary damages for conducting war."
The lawsuit sought unspecified damages.
"We believe the court reached the right result," Justice Department spokesman Brian Fallon said.
Hina Shamsi, director of the ACLU National Security Project, called it a "deeply troubling decision that treats the government's allegations as proof while refusing to allow those allegations to be tested in court."
"The court's view that it cannot provide a remedy for extrajudicial killings when the government claims to be at war, even far from any battlefield, is profoundly at odds with the Constitution," said Shamsi, one of the attorneys who argued the case.
At oral arguments last July, the judge challenged the Obama administration's position repeatedly, pointedly asking "where was the due process in this case?" for the now-dead U.S. citizens targeted in the drone attacks.
When an administration lawyer said there were checks in place, including reviews done by the executive branch, Collyer said "No, no, no, no, no," declaring that "the executive is not an effective check on the executive" when it comes to protecting constitutional rights. But in Friday's ruling, it was clear that the administration's arguments had a strong impact on the judge, who was appointed by President George W. Bush.
The government argued that the issue is best left to Congress and the executive branch, not judges, and that courts have recognized that the defense of the nation should be left to those political branches.
Anwar al-Awlaki's classification as a key leader raises fundamental questions regarding the conduct of armed conflict, Collyer's 41-page opinion stated. The Constitution commits decision-making in this area to the president, as commander in chief, and to Congress, the judge said.
U.S.-born al-Qaida leader al-Awlaki and Samir Khan, an al-Qaida propagandist, were killed in a drone strike in September 2011. Al-Awlaki's 16-year-old son, Abdulrahman, was killed the following month.
The lawsuit was filed by Nasser al-Awlaki — Anwar's father and the teen's grandfather — and by Sarah Khan, Samir Khan's mother
"What I am asking is simply for the government to account to a court its killings of my American son and grandson, and for the court to decide if those killings were lawful," said Nasser al-Awlaki. "Like any parent or grandparent would, I want answers from the government when it decides to take life, but all I have got so far is secrecy and a refusal even to explain."
Anwar al-Awlaki had been linked to the planning and execution of several attacks targeting U.S. and Western interests, including a 2009 attempt on Christmas Day on a Detroit-bound airliner and a 2010 plot against cargo planes.
"The fact is that Anwar al-Awlaki was an active and exceedingly dangerous enemy of the United States, irrespective of his distance, location, and citizenship," said Collyer. "As evidenced by his participation in the Christmas Day attack, Anwar al-Awlaki was able to persuade, direct, and wage war against the United States from his location in Yemen, without being present on an official battlefield or in a hot war zone."