Arson investigators are working to determine what caused a fire that broke out early Friday morning at the Salvation Army warehouse in the River North neighborhood of Denver.

Denver Fire says a call first came in about the blaze at 1:04 a.m. The caller told dispatch they heard an explosion and could smell smoke.

Crews arrived two minutes later, and firefighters say they were able to get the fire under control in 20 minutes.

The fire was contained to the interior of the building, which was filled with smoke and granted firefighters no visibility.

Investigators are working to determine the extent of the damage.

The warehouse is located at 4751 N. Broadway.