For 90 minutes, a tangle of chairs piled up against the classroom door was all that stood between Ohio State student Harrison Roth and an “active shooter” at the university.

The college junior from Plainview and other students shoved what they could against the classroom door, even though they knew the door swung out — easy for the “shooter” to open. A classmate with some sort of military experience had said blocking the sole door into the room was the thing to do, Roth said.

“The chairs would at least slow the attacker enough for us to run,” said Roth, 20. “We wanted to make sure we could do something.”

He snapped a shot of the barricade and tweeted it — a photo seen around the world, leading to interview requests from major U.S. news outlets, as well as those in Mexico, England and Italy.

“I’m safe in a barricaded room,” Roth tweeted. “If you’re on campus, get in a room and stay safe.”

Any other day, Roth would have been walking by the very spot where the attacker drove into pedestrians and started stabbing people before he was fatally shot by police.

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His economics class was in a building diagonally across from the mayhem and he would have been walking home after class, except he was just finishing up an email to his fraternity when he saw the university text about an active shooter.

“This is bad,” he remembered thinking.

Ohio State student Harrison Roth a college junior from Plainview and other students shoved what they could against the classroom door, even though they knew the door swung out - easy for the shooter to open. Photo Credit: Harrison Roth

He didn’t know how bad it was until he learned the major news outlets had descended on the campus.

“I’m sitting there and thinking I’ll be out of here soon and I look over at some student’s . . . [smartphone] screens and I can see CNN has it nonstop,” Roth said.

He looked out of the window and the campus streets were deserted.

Roth texted his friends — they were safe — and when his father called him, he whispered that he was OK.

He listened to the police scanner to hear what was going on and at one point, there seemed to be four active shooters — the suspect who was killed, two people who were arrested and released, and a fourth whom police were seeking in a garage. In a reassuring sign, Roth said, the building where he was barricaded wasn’t mentioned as he listened to the scanner.

When classmates saw how Roth’s Twitter photo had put the world inside their room, they laughed.

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“We weren’t thinking too much about what was going on and how it was so close to us,” he said. “There wasn’t any hysteria. I don’t think it hit any of us until afterward.”

At 11:33 a.m., a university text lifted the lockdown.

Afterward, Roth said, he, his girlfriend and his fraternity brothers at Zeta Beta Tau held a pancake breakfast, where he was glad for the support of his fraternity. Then he and his girlfriend went to a shopping center to take their minds off the attack.

Roth said he tried not to think about whether his life was in peril and was trained for such situations when he was a camp counselor.

“Once you lose your composure,” the junior said, “you have nothing.”