PORTLAND, Ore. -- An overflowing tank dumped thousands of gallons of water into the Portland State Bookstore basement Sunday night, causing $1 million in damage.

On Monday, bookstore workers cleaned up the soggy mess that was caused by a sensor malfunction in the storage tank that holds water for fire suppression.

Ken Brown, who runs the Portland State Bookstore, said the sensors indicated there was little water in the tank when, in fact, it was filled with 40,000 gallons.

"(The storage tank) failed to monitor itself properly and it unloaded a lot of water on us," Brown said.

This marks the fourth flood in the building since it opened in 2001. One previous incident was caused by a similar failure.

Thousands of textbooks stored on lower shelves were damaged and crews tried to salvage clothing and supplies.

PSU students said they were surprised to see the store surrounded by yellow tape Monday morning.

"Yeah, I can't believe it because the basement is where all the textbooks are. So, that's going to be a hell of a job," Karina Dupont, a PSU art student.

Brown said the university and the company that installed the system are investigating the malfunction.

Overflowing Tank Floods PSU Bookstore