Holder's letter says Obama formalized recently the process of selecting targets abroad for drone attacks.
"This week the president approved, and relevant congressional committees will be notified and briefed on, a document that institutionalizes the administration's exacting standards and processes for reviewing and approving operations to capture or use lethal force against terrorist targets outside the United States," the letter says.
Sen. Lindsey Graham, R-S.C., said in February that around 4,700 people have been killed by U.S. drone attacks around the world during the so-called "war on terror." At least one American citizen was killed in an attack before 2009, Kamal Derwish, who was killed in Yemen in 2002.
The American Civil Liberties Union, which is suing the government for documents about the drone program and for information regarding the killing of al-Awlaki, his son and Khan, said in a statement it's optimistic the disclosure will bring even more transparency.
"The government must disclose its still-secret targeted killing memos so the public can determine if they contain criteria as vague and elastic as its definitions of 'imminence' and 'feasibility of capture,'" said Hina Shamsi, director of the ACLU National Security Project, in the statement. "The letter also underscores how little the public still knows about this unlawful program and its consequences, including the previously unknown killing of a fourth American citizen more than a year ago. It does nothing to shed light on the government's legal criteria and factual basis for the killings of thousands of non-citizens – including reportedly hundreds of civilians – in a program that is unlawful, dangerous, and unwise."