BATON ROUGE - Fire investigators are looking into two fires at a four-plex in the 2400 block of Mason Avenue Saturday. 

The cause of the first fire is still unknown. Crews arrived to find the front upstairs unit of the four-plex building fully engulfed in flames. Both upstairs units were completely destroyed and the downstairs units sustained enough heat and water damage to consider them a total loss as well.

A single-family house also sustained minor heat damage from the fire.

The second fire is being investigated as an arson. Firefighters returned to the location to find what was left of the second floor from the first fire. It was determined that the fire is believed to have begun in the rear of the second-floor unit. Crews say it caused significantly more damage to the unit and the first floor.

This remains under investigation.