MISSOULA, Mont. -

Officers are investigating the cause of a small explosion in Missoula Wednesday morning.

The blast happened in an apartment building above Bernice's Bakery on South 3rd Street in Missoula.

Missoula Police say a 46-year-old male resident of the apartment was uninjured and was not in the apartment at the time of the explosion. Another man, who police described as a guest, was inside when the explosion happened. But officers say he left before law enforcement arrived. Police are trying to track him down to check his welfare and possible injuries.

Both the Missoula Police Department and the Missoula Fire Department are investigating the explosion. The building is also being evaluated for safety.

An NBC Montana reporter was on scene shortly after the explosion and said at least one window was shattered.

While the explosion happened just above the bakery, the people at Bernice's are hard a work. They're still making extra treats for Valentine's Day, and are open for normal business hours.

Anyone with information about the incident is asked to contact Missoula Police Cpl. J.C. Denton at (406) 552-6300 or Gordy Hughes at the Missoula Fired Department at (406) 552-6210.