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Video posted online by Syrian opposition activists appears to show the moment one in a series of deadly explosions struck the campus of Aleppo University on Tuesday as students were taking exams. The clip begins with a view of smoke rising from behind a university building as students mill about. Moments later, following a very loud explosion close to the camera, students run for cover and a much larger plume can be seen above the building.

An estimated 52 people were killed in the blasts, with dozens more injured. Once Syria’s commerical epicenter, the city of Aleppo has essentially been the site of almost constant violence since July, with insurgents and government forces locked in a stalemate over control of the city. The university, which is in a government-controlled part of the city, had been conducting classes as fighting went on.

Antigovernment activists said university buildings had been hit by missiles fired by Syrian military forces. But SANA, Syria’s state-run news agency, indicated that the missiles had been fired by the insurgents.

“The most painful scene was a chopped hand with a pen and notebook right next to it,” an education student who identified himself as Abu Tayem said over Skype. “I saw blood, flesh littered all around.”