A bomb detonated in an empty classroom on the first floor of the Yale Law School (YLS) building on Wednesday afternoon, injuring no one but causing substantial damage to the building.

The bombing came on the heels of the decision Tuesday by the Department of Homeland Security to raise the national threat level from elevated to high, represented by the color orange.

According to an announcement posted on the Yale Law School website, the explosion occurred at 4:40 p.m. It destroyed a wall separating the classroom from an alumni lounge next door and damaged portraits of famous alumni that had hung on the wall.

No official has yet linked the attack to international terrorism, and an investigation into the nature of the bomb is still ongoing.

Rare books housed directly below the classroom may also have been damaged when the sprinkler system was activated, according to the announcement.

Harvard University Police Department (HUPD) spokesperson Steven G. Catalano declined to comment on how Harvard’s security would respond to the bombing, emphasizing HUPD’s practice of not commenting on security.

“To date, the HUPD has not received any credible threats made against the University,” he said. “We will continue to work with our partners at the city, state, and federal level to assess the threat level at Harvard.”

Yale Law School is housed in one large complex, which was evacuated after the blast and has remained closed, separating many students from their belongings and dorm rooms in the midst of final exams week. Many law students are being housed temporarily in undergraduate dorms.

Second-year YLS student Inayat H. Delawala was studying for an exam on the third floor of the building, directly above the classroom, when he said he heard a “thunderous boom that just reverberated down the reading room.”

“It shook the place, and you could tell it was an explosion or something serious,” he said.

Ting Wang ’02, a first-year student at YLS, said he was in his bedroom, in another wing of the building when the bomb exploded. He said that while students closer to the explosion described feeling the building shake, he and his neighbors were initially unaware of the incident.

“At one point the power was cut,” he said. “Nobody knew what was going on... somebody said there was a fire next door.”

Wang was still unaware of the severity of the incident when he and his neighbors were evacuated by police officers.

“I just left with a t-shirt on my back and a book in my hand,” Wang said. “I thought I’d be back in 20 minutes.”

Wang finally found out that there had been an explosion when he began receiving phone calls asking if he was all right.