BOZEMAN, Mont. -

A blaze overtook a Belgrade apartment Wednesday and firefighters say they are suspicious.

NBC Montana was on-scene Wednesday evening and saw investigators still combing the scene for clues. It happened on the 700 block of Minnesota Street in Belgrade.

Fire crews responded just after noon and found the upstairs of the duplex engulfed in flames. No one was home at the time, but residents were downstairs and everyone made it out safely.

Investigators were looking closely at a mattress, but could not tell us where where or how the fire started. Officials say they suspect foul play.

"They'll be going back, pin-pointing the cause of origin, looking for any accelerants -- any reason why it could have been a malicious fire started. And in determining, ruling out false leads, what the actual cause was," said Central Valley Fire Chief Ron Lindroth.

Officials say the apartment is heavily damaged, but the downstairs apartment is OK.

The Gallatin County Sheriff's Office, Belgrade Police and Central Valley Fire are still investigating the scene. We will continue to keep you updated as we learn more information.