DALLAS — The authorities in Texas and North Dakota said on Friday that they had found no explosives after two large public universities were evacuated over bomb threats — false alarms that forced tens of thousands of students and staff members to flee in the middle of classes at the University of Texas at Austin and at North Dakota State University in Fargo.

In Austin, William Powers Jr., the university’s president, said at a news conference on Friday that campus buildings had been checked and that he was confident the campus was safe. But while the university did allow access to buildings Friday afternoon, officials canceled classes for the day.

At North Dakota State, which has 14,000 students, school officials ordered an evacuation Friday morning after receiving a separate bomb threat, but had reopened campus by 1 p.m., officials said. Classes were to resume at 2 p.m.

It is not clear who made the threats or whether they were connected, officials said.

At the University of Texas, a message posted on the university’s Web site said: “Evacuation due to threats on campus. Immediately evacuate ALL buildings and get as far away as possible. More information to come.”