New London — The city’s fire marshal said a sprinkler system helped to avert what could have been catastrophic fire Wednesday at the Connecticut Waste Processing Materials facility.

Fire Marshal Vernon Skau said the cause of the early morning fire will remain undetermined because of the amount of construction debris at the 45 Fourth St. transfer station and difficulty in pinpointing the fire’s origin.

Firefighters had responded to a 3 a.m. fire alarm at the facility to find smoke showing at the large-scale transfer station, which moves bulky waste by the connected rail line. Firefighters described the two-alarm fire as “deep-seated” in the construction material. CWPM used its equipment to move the piles to allow firefighters, on scene for about four hours, access to the fire and move the debris.

Skau said the fire was held in check by the sprinkler system until firefighters arrived. The building was not damaged.