FOND DU LAC, Wis. (CBS 58) -- Fond du Lac Fire Rescue responded to an apartment fire above The Bull Pen tavern, reportedly receiving heavy damage on Wednesday, Jan. 4.

The fire response team made their way to 243 W. 9th St. for a fire call at around 10 p.m. They found what is described as "heavy fire" showing from a second-floor window.

Crews searched the building for occupants once the flames were extinguished, but found no one inside. They did, however, find several pets that perished during the fire. Fond du Lac Fire Rescue has not yet revealed how many pets' lives were lost.

Investigators are working to determine the cause and origin of the fire.