Five years after the U.S. killed American-born imam Anwar al-Awlaki, he is still inspiring jihadists at home and abroad. Here's everything you need to know:

Who was he?
Anwar al-Awlaki was originally the face of moderate Islam in America. He was born in New Mexico to Yemeni immigrant parents, but his parents moved back to Yemen when he was 7, and the family didn't return to the U.S. until he was in college. He was a young imam at a large mosque in the Washington suburb of Falls Church, Virginia, when the 9/11 attacks propelled him into the spotlight. The media needed Muslim leaders to respond to the attacks and explain Islam to viewers, and the articulate and charismatic Awlaki was a natural. He gave numerous TV and newspaper interviews and even addressed an audience at the Pentagon, condemning terrorism while also criticizing U.S. foreign policy in the Muslim world. "We are the bridge between America and 1 billion Muslims worldwide," he told his followers.

Why did he turn on the U.S.?
The FBI started following Awlaki soon after it learned that two of the 9/11 hijackers had prayed at both his Virginia mosque and a San Diego mosque where he had preached years earlier. They found no evidence that Awlaki had any direct involvement with terrorism. But they did discover his frequent visits to prostitutes, and they kept an extensive file on those visits. Scott Shane, author of a book on Awlaki called Objective Troy, reports that the manager of one of the escort services called the imam in 2002 to warn him that the FBI knew everything. Awlaki was outraged the FBI had been surveilling him, and feared he would be exposed as a hypocrite; a married man, he'd often denounced "the moral decay" of the West. So he quickly fled to London and later to Yemen, where he was further radicalized. By 2007, he had declared loyalty to al Qaeda there and become the terrorist group's chief propagandist, recording sermons denouncing the U.S. and calling on Muslims to massacre Americans. In 2011, he was killed in a targeted U.S. drone strike (see below). But Awlaki's death didn't end his influence.

Who has been influenced by him?
Just about every Islamist who has attacked the U.S. since 9/11. He had direct email contact with Nidal Malik Hasan, who killed 13 people in a 2009 shooting attack at Fort Hood, Texas. He helped recruit and train Umar Farouk Abdulmutallab, who in 2009 tried to blow up an airplane over Detroit with a bomb in his underwear. Awlaki died before the rise of ISIS, but nearly every ISIS adherent who speaks English has seen his lectures on YouTube or read his articles in Inspire, the online al Qaeda magazine he helped launch. Syed Rizwan Farook, who along with his wife killed 14 people in a 2015 rampage at his workplace in San Bernardino, California, watched Awlaki sermons; so did Mohammod Youssuf Abdulazeez, who killed four Marines and a Navy sailor in Chattanooga, Tennessee, also in 2015. The Tsarnaev brothers, who bombed the Boston Marathon in 2013, were fans of the videos, and plans for the pressure-cooker bomb they used can be found in Inspire's article "How to Make a Bomb in the Kitchen of Your Mom." Ahmad Khan Rahami, the man accused of planting a bomb that injured two dozen people in New York's Chelsea neighborhood last month, may have followed the Inspire plans as well, and his father spoke to the FBI in 2014 because he was concerned that his son was spending so much time watching Awlaki videos.

What's so seductive about these videos?
Awlaki's lectures are smart and accessible. Speaking idiomatic English with just a trace of an Arabic accent, he comes across as an engaging, thoughtful preacher — not ranting, but backing up his assertions with scholarly analysis of the Quran and ample references to modern life and current politics. Most importantly, he taps into the anger many Muslims feel over the enormous civilian death toll stemming from the invasions of Iraq and Afghanistan, portrays Muslims as victims of Western oppression, and channels this resentment into a call for vengeance. In the notorious lecture "Call to Jihad," Awlaki calmly explains why it is the duty of every Muslim to kill Americans.

Can his lectures be banned?
There are thousands of hours of Awlaki lectures on YouTube alone. In 2010, under pressure from Congress, YouTube voluntarily removed hundreds of Awlaki videos, but many people have copies, and a few months later they were all back up. Software exists to find and delete child porn, and experts say a similar bot could be deployed against Awlaki's image and voice, but the government hasn't taken that step, fearing it would be a violation of the First Amendment. "It's not a technical problem," says computer scientist Hany Farid. "It's a policy issue." So even in death, Awlaki continues to haunt the country where he was born. "The half-life of Awlaki's message is amazing," said Patrick M. Skinner, a former CIA counterterrorism officer. "It really is like plutonium. It's toxic, and it doesn't go away."

Assassinating a U.S. citizen
Yemeni intelligence found Awlaki's hideout in that country's tribal badlands in 2011, but the Obama administration decided it would be impossible to take him alive. Instead, President Obama ordered a drone strike, making Awlaki the first American citizen to be targeted and killed by the U.S. government without due process since the Civil War. Two other U.S. citizens, New York native Samir Khan, editor of Inspire, and Awlaki's 16-year-old son, Abdulrahman, were also killed by drone strikes in Yemen. Obama said the killing of Awlaki was both morally and legally justified, on the grounds that he was fighting for the enemy during wartime and posed "an imminent threat" to Americans. "When a U.S. citizen goes abroad to wage war against America," said Obama, "his citizenship should no more serve as a shield than a sniper shooting down on an innocent crowd should be protected from a SWAT team." The American Civil Liberties Union sued, saying Awlaki had not been convicted of any crimes, and that the killing violated the Constitution's guarantee of due process. But a federal court dismissed the case on national security grounds.