WINSLOW — The Office of State Fire Marshal is investigating after a fire tore through a warehouse where recycled paper was stored at the Kennebec River Development Park earlier this week.

The cause of the fire at 100 Benton Ave. is still undetermined, as crews have yet to reach the location where it started, according to Winslow’s Deputy Fire Chief Michael Murphy.

Murphy said firefighters face a difficult cleanup process due to the nature of the warehouse’s contents. Bales of recycled paper each weighing thousands of pounds were stacked atop one another within the complex, along with tubs of shredded paper.

“We still haven’t dug that corner out yet, there’s still lots of material in there,” Murphy said Friday.

The fire was first reported around 10 a.m. on Wednesday. Crews had it under control by 2:30 p.m., though Murphy said it continued burning well into the evening. The Office of State Fire Marshal has launched an investigation into the incident.

The warehouse is currently rented by Huhtamaki to store recycled paper. The complex, a former paper mill once owned by Scott Paper Co. and then Kimberly-Clark Corp., is currently owned by Marden’s Surplus & Salvage, which uses it primarily for storage and shipping.

The warehouse where the fire occurred is equipped with a sprinkler system that reportedly helped control the blaze as firefighters worked to gain entry into the building.

Crews returned to the steel-sided warehouse around 8 p.m. Thursday after the fire rekindled , Murphy said. They remained for about an hour before clearing the scene again shortly after 9 p.m.