ST. PETERS — Two firefighters with Central County Fire & Rescue who were inside a burning shed Thursday morning fell into a maintenance pit filled with water and debris and were slightly injured.

One of the firefighters was treated and released from a hospital; the other had minor injuries and remains hospitalized for additional treatment, said Central County's Assistant Fire Chief Steve Brown.

The one-alarm blaze about 3:45 a.m. Thursday was on the property of Hackmann Brothers, a heating oil supplier at 2929 North St. Peters Parkway. Firefighters arrived to find a 30-by-40 foot metal shed engulfed in flames. A woman who lives in a home on the Hackmann site, adjacent to the fire, got out safely, Brown said.

The cause of the fire is under investigation.

Firefighters had the blaze under control in about 45 minutes. The shed is used for maintenance and storage. Petroleum-based products were inside.

A maintenance pit built into the shed's floor was about five or six feet deep, and the firefighters weren't aware it was there, Brown said. After they fell, the firefighters sounded a "mayday" call, which was an intense few moments inside the burning structure, Brown said. One firefighter was able to get out on his own, and the other was pulled to safety, Brown said.

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