PHOENIX (3TV/CBS 5) – Investigators are trying to determine what sparked a two-alarm fire at a Phoenix recycling plant Sunday morning.

Crews were called out to the area of 65th Avenue and Buckeye Road at about 10 a.m. According to Captain Rob McDade of the Phoenix Fire Department, “large bails of recycled cardboard and other paper materials” were burning, causing the flames to spread quickly.

McDade said firefighters went up to the roof of the large building to open skylights to vent the smoke.

Crews knocked down the fire and were dealing with hotspots in the building less than two hours after the first call.

The Phoenix Fire Department escalated the call to a second-alarm, which means more firefighters and resources were sent to the scene, “because of the size and contents inside of the structure,” McDade explained.

No injuries were reported.