Scott Shane

When a lonely Virginia teenager named Ali Amin got curious about the Islamic State last year and went online to learn more, he found a virtual community awaiting.

During the 1980s and '90s, the historic alliance between the wealthy monarchy of Saudi Arabia and the country's powerful clerics emerged as the major financier of international jihad, channeling tens of millions of dollars to Muslim fighters in Afghanistan, Bosnia and elsewhere. Among the project's major patrons was Prince Salman bin Abdul-Aziz Al Saud Salman, who last month became Saudi Arabia's king.

Aware that intensified American counterterrorism efforts have made an ambitious Sept. 11-style plot a long shot, al-Qaida propagandists for several years have called on their devotees in the United States to carry out smaller-scale solo attacks and provided the online education to teach them how.