DENVER (KDVR) — A Saturday fire at an abandoned motorsports park in Arapahoe County is being investigated as arson, according to the Arapahoe County Sheriff’s Office.

ACSO said 70 firefighters from 13 agencies responded to the blaze at the old Colorado Motor Sports Park, located at 2050 S. County Road 201.

The fire was on the second story of the building, and ASCO said it does not know how it started.

However, ACSO tweeted Saturday calling it a “suspicious fire” with a high probability that it was human-caused.

ASCO said it would hold the scene overnight and the arson investigation continued Sunday.

The cause of the fire was still unknown as of Sunday afternoon.