More than 1,500 Ohio State students had to leave class Tuesday morning because of a bomb threat that led university officials to close four buildings on campus.

Despite the university’s Buckeye Alert system, which is meant to notify subscribers of emergencies, many students were unaware of the threat that closed the William Oxley Thompson Memorial Library and McPherson, Smith and Scott laboratories.

University officials announced at 8:41 a.m. Tuesday that police were investigating bomb threats made to the four buildings. Officials evacuated and cordoned off the buildings.

The evacuations came about an hour after someone sent an anonymous message to the FBI tip line, FBI Special Agent Michael E. Brooks said.

The tip went through the FBI headquarters in Washington, D.C., before going to FBI field officers in Columbus, who notified OSU officials of the threat at 8:19 a.m. There was about a 20-minute lag between the time university officials learned of the threat and the time they sent messages to Buckeye Alert subscribers.

Officials from several law enforcement agencies searched the four buildings throughout the day with bomb-sniffing dogs. All classes and activities in those buildings were canceled during the searches, but all other campus activities continued as usual.

Officers did not find any explosives, and their search concluded at about 8 p.m. Tuesday. All buildings were reopened by 9 p.m.

Following the investigations Tuesday, OSU officials sent a university-wide e-mail encouraging students to subscribe to the Buckeye Alert system.About 32,000 subscribers to the system received warnings of the situation Tuesday morning, said Bob Armstrong, director of Emergency Management and Fire Prevention.

But some who enrolled in the opt-in alert system said they never got the message.

“I signed up my freshman year, and I’ve gotten other alerts from OSU, but I didn’t get one today,” said Allie Dorsky, a fourth-year in strategic communication. “The only e-mail I got today was the Student Life e-mail that was sent out later.”

Dorsky is the vice president for risk management for Alpha Chi Omega sorority and is responsible for keeping the chapter informed of safety issues.

“I was pretty shocked when I found out about the bomb threat. It wasn’t just some minor thing,” Dorsky said. “It’s pretty alarming that something like that was going on around me, and I had absolutely no idea.”

Dorsky said some classmates who also subscribe to Buckeye Alert did not receive warnings.

Armstrong said those cases might not be the university’s fault.

“It’s not always an issue with our system,” he said. “Once we send out the text to the carrier, it’s up to the carrier, such as Verizon, to distribute the text to the individual.”

But Dorsky said she was signed up to receive e-mail alerts, so no carrier was involved.

Undergraduate Student Government President Micah Kamrass will meet with Public Safety officials Friday and will urge them to switch the Buckeye Alerts to an opt-out system, so students will automatically be enrolled unless they unsubsribe.

USG has been pushing for that policy for about two years, Kamrass said. He said he will use the bomb threats Tuesday as an example of why the university should automatically enroll students in the system.

“I hope this example shows why that’s so important,” Kamrass said. “In matters like this, that can literally be life and death.”

Kamrass said he also had heard complaints from students who did not receive notice of the bomb threat.

Some students who had to leave class because of the bomb threat said they did not know what was going on. Kevin Haimes, a fourth-year in mechanical engineering, said the fire alarm went off about 10 minutes into his 8:30 a.m. engineering class in Scott.

“Basically, everyone thought it was a fire drill, so we just casually walked outside,” Haimes said. “Everyone was just hanging out outside for five to 10 minutes before one police woman came up to us and told us to back up.”

The group stayed close to the building for about 30 minutes, Haimes said.

“We were in the part they ended up roping off until they roped it off,” he said.

Stanley Durkin, professor of physics, was teaching an 8:30 a.m. class in Smith when the alarm went off. He said nobody knew what was happening.

“It took us about 30 minutes to know what was going on,” Durkin said. “After about 30 minutes, police moved in and said nobody could stand on the sidewalk outside Smith Lab.”

Haimes left his belongings in the building during the evacuation and was still waiting to retrieve them at about 4 p.m. Tuesday. He spoke with a police officer outside at about 2 p.m., but the officer could not answer his questions.

“We asked if they were going to search our bags or if they were going to move them somewhere,” Haimes said. “The police officer who was standing out there didn’t have any idea what was going on.”

At about 5 p.m., a student notified police about an unattended backpack with a wire sticking out of it in Macquigg Laboratory. Police cleared the first floor of Macquigg for about 20 minutes but did not find any explosives.

Anything left in McPherson, Scott and Smith will go to the OSU Police Department in Blankenship Hall and can be picked up from 8 to 11 a.m. and noon to 2 p.m. today, according to a Public Safety release. Property left in Thompson will go to the west security desk and can be retrieved during regular library hours.

Molly Gray, Lauren Hallow, Kelsey Buller, Collin Binkley and Bethany Brakemeyer contributed to this story.