Most recently, Facebook, Twitter and YouTube all admitted that the Russian government had used their platforms last year to pose as American voters and spread divisive propaganda. Under fire from Congress, all the companies pledged to police their sites more carefully in the future.

The prolific presence on YouTube of Mr. Awlaki, who in his later years called for attacks on America and Americans, has been a subject of complaints since November 2009, when he wrote on his website that Army Maj. Nidal Malik Hasan was a “hero” for fatally shooting 13 people at Fort Hood, Tex.

Born in New Mexico to Yemeni parents, Mr. Awlaki spent half his life in the United States and spoke excellent English. As a popular young imam in Denver, San Diego and a Washington, D.C., suburb, he recorded lecture series that were eventually best sellers among Western Muslims — notably his 53-CD boxed set on the life of the Prophet Muhammad.

After leaving for London in 2002, however, he gradually embraced the view that the United States was at war with Muslims, who had a religious duty to fight back. After evidence emerged that he had recruited and coached the young Nigerian who tried to blow up an airliner over Detroit on Christmas Day in 2009, President Barack Obama ordered a legal review and then placed him on the terrorist kill list.

Mr. Awlaki and three other members of Al Qaeda were killed in a drone strike in Yemen in September 2011. But his death on Mr. Obama’s orders made him a martyr in the eyes of many admirers, who posted more and more of his work on YouTube.

Much of it was the early, mainstream material, which clearly did not violate YouTube’s “community guidelines.” But especially after Mr. Awlaki’s death, fans often labeled even his early material as the work of the martyr killed by America. The number of videos on YouTube presenting or celebrating his work more than doubled from 2014 to 2017, even as investigators found his decisive influence in most of the major terrorist attacks in the United States and some in Europe.