A rainy Friday morning at the University of Texas, Austin dissolved into widespread alarm when a man claiming to be associated with al-Qaida called at 8:35 a.m. and announced he had planted bombs throughout the 50,000-student campus.

The bombs, he said, would go off in 90 minutes.

Sirens wailed and students' phones pinged with text messages as campus staff moved swiftly to evacuate the buildings.

"One of them said to me 'get off this campus as soon as possible,'" said Elizabeth Gerberich, an 18-year-old freshman from New Jersey.

Students at first through the sirens might be part of a test the school does once a month.

"Then we started to tell everybody, 'No actually we have to get out of here pretty immediately'...there was definitely a little bit of nervous tension," said Abby Johnston, 22.

Tania Lara, a graduate student at the Lyndon B. Johnson School of Public Affairs, was at work inside a central campus academic building when she got a text message to get as far away as possible.

"It was calm but nobody knew what was going on," she said, describing a crush of students heading for the exits. "No one was yelling 'get out of here' or anything like that."

The 90-minute deadline passed without incident. Administrators cancelled classes for the remainder of Friday, but other university activities were scheduled to resume by 5 p.m.

"We are extremely confident that the campus is safe," UT President William Powers said at an early afternoon news conference.

The University of North Dakota received a similar threat. Investigators found no bombs on either campus by early afternoon. School officials have not determined whether the threats are related.

The Associated Press contributed to this report.