The U.S. government has the authority to kill terrorist suspects, even if they're American citizens, according to a new Justice Department memo.

In 2011, a drone strike in Yemen that killed Anwar al-Awlaki and Samir Khan, both U.S. citizens, set off a controversy over U.S. policy for drone attacks.

Since then, the Obama administration has been searching for legal justification for its drone strikes against U.S. citizens who have links to al Qaeda.

The new memo provides a much broader definition of whether those Americans pose an imminent threat.

Now the government only needs to show that the targeted suspect is involved in ongoing plotting against the United States.

The American Civil Liberties Union said the document is "profoundly disturbing."

"It's hard to believe that it was produced in a democracy built on a system of checks and balances," the ACLU said.