The disclosure came in a letter U.S. Attorney General Eric Holder sent to congressional leaders, The New York Times reported.
Holder said the administration deliberately targeted Anwar al-Awlaki, a radical Muslim cleric and American citizen killed in a drone strike in September 2011 in Yemen. The letter said U.S. drones also killed U.S. citizens Samir Khan, who died in the same strike; Awlaki's son Abdulrahman al-Awlaki, who also died in Yemen; and Jude Mohammed, who died in a strike in Pakistan, but they were "not specifically targeted," Holder wrote.
The newspaper noted the disclosure came the day before President Obama is to make a major speech on national security issues.
Holder said last year the administration maintains it is legal to target American citizens deemed to be operational terrorists, who pose an "imminent threat of violent attack" and whose capture is infeasible. The legal rationale also was presented in an unclassified white paper given to Congress last year and became public this year.
Holder's letter said it wasn't Awlaki's words urging violent attacks against Americans that led the United States send its drones after him but his direct involvement in planning attacks.
Holder alleged Awlaki planned the attempted bombing of a Detroit-bound airliner on Dec. 25, 2009, and played a key role in an October 2010 plot to bomb cargo planes bound for the United States, the Times said.
"Moreover, information that remains classified to protect sensitive sources and methods evidences Awlaki's involvement in the planning of numerous other plots against U.S. and Western interests and makes clear he was continuing to plot attacks when he was killed," Holder wrote.
"The decision to target Anwar al-Awlaki was lawful, it was considered, and it was just."