A string of bomb threats caused thousands of people to be evacuated from three campuses across the nation Friday.

After necessary sweeps were conducted, however, the threats were deemed bogus.

North Dakota State University Fargo campus, University of Texas at Austin and a smaller Hiram college in Ohio were the three universities that received similar bomb threats, which ended in mass evacuation.

The first threat was made to UT-Austin as early as 8.35 a.m., when a man claiming to be from the Al-Qaeda said bombs were placed all over the campus and would go off in 90 minutes. Administrators waited for approximately an hour before sounding off sirens throughout the campus, informing students to get 'as far away as possible' through emergency text messages.

Only after an all-clear message was issued by noon, did students return to the campus which hosts around 24,000 faculty and 50,000 students.

But the wait of one hour did not go down very well with many. Though authorities say they started the search for explosives before the sirens went off, many students say the one-hour wait was too risky, especially in the wake of U.S. embassies getting mobbed and attacked in Islamic nations.

At a press conference following the reopening of the campus, UT-Austin president William Powers defended the one-hour wait. He said the school authorities were carefully evaluating the threat.

"It's easy to make a phone call ... the first thing we needed to do was evaluate," Powers said. "If the threat had been for something to go off in five minutes, then you don't have the time to evaluate, you just have to pull the switch."

Now, after the evaluation and the complete search, he said he is 'extremely confident the campus is safe.'

North Dakota State University too received a similar threat 'of an explosive device' at 9.45 a.m., said FBI spokesman Kyle Loven to AP.

President Dean Bresciani said about 20,000 people left the campus as part of an evacuation 'that largely took place in a matter of minutes.'

The evacuation took place in an orderly manner and students did not panic, police officials say.

Hiram College, Ohio, which is about 35 miles southeast of Cleveland with a strength of1,300 students, received an emailed bomb threat about 4 p.m. and an immediate evacuation order was followed.

Hiram spokesman Tom Ford said safety teams with bomb-sniffing dogs checked 'room by room, building by building' on campus.

Valparaiso University in Indiana also reportedly received unspecified threats, but officials said they were 'substantially different' from threats experienced by other schools, reports star-telegram