An explosion that occurred after a top blew off a water cooler Thursday morning at the Foster-Walker Complex left two Northwestern maintenance engineers injured.

Fire trucks had just left the Plex, 1927 Orrington Ave., after responding to a false alarm caused by a faulty smoke detector in a first floor lounge around 10 a.m., Evanston Fire Division Chief Alan Berkowsky said.

But they made a U-turn and returned to the Plex around 10:20 a.m. to respond to the “ruptured” water cooler and two hurt people, Berkowsky said.

The two maintenance engineers were taken to Evanston Hospital.

One man, 44, was treated for back pain and the other man, 45, was treated after being struck in the face by an object when the water cooler blew.

The water cooler normally provides air conditioning to parts of the building.

Mark Mitchell, safety manager for the Office of Risk Management, said the maintenance engineers were changing a filter when the cooler malfunctioned and the filter failed.

There was a rapid release of chilled water and the top came off of the cooler, Mitchell said.

He emphasized this was “not a planned removal.”

Mitchell also said NU would be monitoring the health of the two injured workers.