MSNBC Samir Khan (left) and Anwar al-Awlaki, both U.S. citizens, were killed in in Yemen by an American drone strike.

Of the scores of people dubbed terrorists who have been targeted by American military drone strikes, three men -- all killed in the fall of 2011 -- were U.S. citizens.

And their lives illustrate the complexity of the issue, recently brought to light amid a newly discovered government memo that provides the legal reasoning behind drone strikes on Americans.

Anwar al-Awlaki and Samir Khan were killed by a missile strike in Yemen on Sept. 30, 2011, while al-Awlaki’s son, Abdulrahman, was killed in the country just weeks later.

Since the attacks, family members have called the deaths unjust and sued the U.S. government, calling the killings unconstitutional.

Anwar al-Awlaki, born in New Mexico, became well known for his fiery anti-American sermons posted throughout the Internet.

Samir Khan, who'd lived in both New York and Charlotte, N.C., produced a magazine called “Inspire” that became known for its extreme jihadist views.

But the most controversial drone strike took place on Oct. 14, 2011, when 16-year-old Abdulrahman was killed by U.S. forces.

Family of the Denver-born teenager say he had no ties to terrorist organizations and was unjustly targeted because of his father.

Nassar al-Awlaki, grandfather of Abdulrahman and father to Anwar, said he tried to protect his grandson as Anwar al-Awlaki’s profile grew.

In December, Nassar al-Awlaki told CNN, “In Anwar it was expected because he was under targeted killing, but how in the world they will go and kill Abdulrahman. Small boy, U.S. citizen from Denver, Colorado.”

Nassar al-Awlaki said his grandson snuck out of their Yemen home one night, leaving a note for his mother saying he would return in a few days. The boy never returned, killed instead while eating at an outdoor restaurant.

“Since the issue regarding Anwar came, I tried to insulate the family of Anwar from everything, regarding this matter,” Nassar al-Awlaki told CNN. “I took care of him, and suddenly after 2 year absence from his father, he decided to go to our government in Yemen to seek information from his father. That was the only reason he went, and he did not tell us.”

The Obama administration has remained mostly mum regarding Abdulrahman's death, and at times has struggled to explain it.

Read more: Memo details legal case for drone strikes

"I would suggest that you should have a far more responsible father if they are truly concerned about the well-being of their children," former White House spokesperson Robert Gibbs said to a gaggle of reporters in October. "I don't think becoming an al-Qaeda jihadist terrorist is the best way to go about doing your business."

During his presidential campaign, Republican Rep. Ron Paul criticized the killing of Anwar al-Awlaki, saying: “Al-Awlaki was born here, he is an American citizen. He was never tried or charged for any crimes. No one knows if he killed anybody. ... But if the American people accept this blindly and casually that we now have an accepted practice of the president assassinating people who he thinks are bad guys, I think it's sad.”

Anwar al-Awlaki’s ties to the United States go back to his father Nassar, who came to the country to earn a master’s degree. His son was born in New Mexico, and though the family returned to Yemen, Anwar al-Awlaki came back to the U.S. for college, eventually becoming an iman.

Shortly after the Sept. 11, 2001, he became a popular spokesman for moderate Islam, and was often used to juxtapose perceptions that Islam is a religion that spreads hate. But less than a decade later, he was hiding in Yemen as a name on the CIA's kill list.

“I eventually came to the conclusion that jihad against America is binding upon myself just as it is binding on every other Muslim,” he said in an audio message in March 2010.

Conversely, Khan was never interested in the peaceful side of Islam. The New York Times reports that as a teen, Khan’s attraction grew exponentially to militant sites on the Internet after 9/11. Parental concerns and intervention from community leaders proved unsuccessful. Khan was 25 when he died in Yemen.

In July 2012, Samir Khan’s mother, Sarah, joined Nassar al-Awlaki in a lawsuit against four senior national security officials.

“I don’t really necessarily agree with some of the things Anwar said against the United States, but does that mean they should kill him outside the law?” asked Nassar al-Awlaki.

A secretive memo from the Justice Department, provided to NBC News, provides new information about the legal reasoning behind one of the Obama administration's controversial policies. Now, John Brennan, Obama's nominee for CIA director, is expected to face tough questions about drone strikes on Thursday when he appears before the Senate Intelligence Committee. NBC's Michael Isikoff reports.

Update: A fourth American-born citizen, Kamal Derwish, was killed by predator drone in Yemen in 2002. Derwish was not the primary target of the strike, but was riding in an SUV carrying an al-Qaida leader.