Investigators are trying to figure out what caused two 18-wheelers to catch fire Wednesday evening.

It started at a pipe yard on Highway 90 and Miller Road in Sheldon around 6:30 p.m. Wednesday.

Firefighters said they used extra caution because of the chemicals located near the flames.

Officials said two big rigs were destroyed by the fire.

No one was hurt.        

A Harris County Fire Marshal’s Arson Unit is investigating.