Some Iranian citizens traveling to Santa Clara this weekend for a reunion of graduates and professors from a Tehran university have been stopped at U.S. airports and told the visas they were carrying had been revoked.

The State Department on Thursday refused to say why some Iranians coming to the conference with visas approved by U.S. consulates abroad are being denied entry, detained overnight and then sent back to Iran. Event organizers said they know of 20 who have so far been turned away. More than 100 additional Iranian nationals are due at the weekend event.

"We knew that it was going to be difficult to get U.S. visas for the Iranians," said Fredun Hojabri of San Diego, founder of the Sharif University of Technology Association, a worldwide group of former students and professors associated with the 40-year-old school. But Hojabri said over the course of four months he had helped about 150 prospective participants at this weekend's combination reunion and conference work through the process to get visas.

Among those whose visas were revoked was Ali Edrissi, a doctoral student at Sharif University. Bahman Pouranpir, Edrissi's uncle and an industrial engineer from Irvine, waited for about six hours at Los Angeles International Airport on Tuesday for Edrissi and his bride, Sara Nadimi.

"We were so panicked and so shocked, I didn't know what to do," said Pouranpir. Edrissi was put on a flight back home on Wednesday.

"I think before that USA was a very law-abiding country and they gave rights to someone," Edrissi said in a telephone interview from his home in Tehran on Thursday night. "After, I never go to USA anymore. I will never. They give me 1 million dollars, I will not go to USA because that was a bad experience."