(CNN) Mitchell Lee was in an apartment across the street from the dorms at the University of Nevada, Reno, on Friday when his building shook.

At first, the university senior "thought it was like an earthquake or car crash," Lee told CNN affiliate KRNV

Across the street, something had exploded in the basement of a seven-story residence hall, causing major damage to the building where summer school students were staying.

Authorities say there were two explosions -- a small one that prompted a 911 call to the fire department and a larger one after firefighters arrived that blew out a side wall of the building.

Investigators are still working to determine the cause of the explosions, but it is believed there was a mechanical failure in the building, said Steve Leighton, an operations chief with the Reno Fire Department. The campus police department referred to the second explosion as a "major utility incident."

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