from 17,700 to 77,100, DHS figures show.

In order to safeguard the kingdom's image, Saudi diplomats often hurried to repatriate students or others who got into trouble

Although both countries pledged to screen the visa applicants rigorously, some FBI veterans saw the process as inadequate. "They interviewed people, they tried to do some due diligence, but it wasn't much," one former agent said.

The Saudi government also had its own screening procedures to identify militants, slackers, and potential critics of the royal leadership, officials said. Before and sometimes after arriving in the United States, the students were briefed extensively on American social mores, with an eye to helping them to adapt to a culture radically different from their own.

A list of rules for Saudi students abroad, published by the Cultural Missionin 2017, forbids any political or religious discussions or any interviews with local news media. The students are also instructed that if they get into any trouble, or witness another Saudi student doing so, they must contact the Cultural Mission or one of the students who act as its semiofficial representatives on many college campuses.

"The Cultural Mission does keep tabs," Smith said in an interview. "They have representatives at each of the colleges and universities, and they want to make sure that everyone behaves. They run a pretty tight ship."

According to former US national-security officials and Saudi students, the associations of Saudi students on many campuses act as an informant network for the kingdom's intelligence services, reporting to Cultural Mission handlers or intelligence officers working under diplomatic cover.