The students, who were mostly headed to schools in the University of California system, had visas in hand when they were blocked from their flights this month.

At least a dozen Iranian students who were set to begin graduate programs in engineering and computer science say their visas were abruptly canceled and they were barred from their flights to the United States this month. The sudden batch of visa cancellations, which came at a time of heightened tensions between the United States and Iran, set off a scramble by university officials, lawmakers, the students’ union and Iranian-American advocates to figure out what had happened. The State Department said that there had been no change in policy regarding student visas, and higher education officials say that visa problems arise every fall for some of the hundreds of thousands of international students who travel to attend American colleges and universities. But the students, most of whom were headed to schools in the University of California system, say their visas were revoked at the last minute, without any warning or explanation. Most were prevented from boarding flights in Iran, and others from boarding connecting flights in the Persian Gulf. One was detained at Boston Logan International Airport and then turned back.

Many of the students said that a State Department webpage showed their visa cases had been updated around Aug. 30, and they were prevented from boarding in early September. All of that came before a Sept. 14 attack on two key Saudi oil installations, which has escalated a standoff between the United States and its ally Saudi Arabia against Iran.

[President Trump announced a new round of sanctions against Iran on Friday.] A law enacted in 2012 under President Barack Obama requires the United States government to deny visas to Iranian students whose coursework would prepare them to work in the energy or nuclear sectors in their home country. Consular officials have wide discretion on how to interpret the statute and put it in place, said Jamal Abdi, the president of the National Iranian American Council, a Washington-based group. Mark Dubowitz, the chief executive of the Foundation for Defense of Democracies, also based in Washington, said he appreciated that the 2012 law had sought to prevent knowledge gained in the United States from being used in the service of the Iranian government. But he pointed to the difficulty in predicting how students would use technical skills that are widely sought after and applicable in many industries. He suggested a more radical approach: to overturn the Trump administration’s travel ban and require Iranian students in sensitive fields to stay in the United States after graduation.