One D.C. firefighter was fatally injured and three others were also injured early this morning in a two-alarm blaze that heavily damaged a building that has figured prominently in efforts to clean up the downtown 14th Street NW corridor.

The firefighter, who was taken unconscious to George Washington University Hospital from the blaze at 806 14th St. NW, was not identified immediately. Officials said he was injured in a fall from the building's marquee into the basement while trying to gain entry.

A police lieutenant said authorities were investigating the possibility that a kerosene heater may have caused the blaze in the building, which houses the Casino Royal theater.

The theater, which is known for adult films, was in operation when the blaze broke out about 1 a.m. A security guard alerted patrons and all 20 were believed evacuated safely, according to authorities.

Within a few minutes flames raged through the interior of the two-story building.

The 806 14th Street NW address is part of a parcel sold earlier this year by Herbert C. Cole for about $5 million to a partnership headed by developer Dominick F. Antonelli Jr. Cole, who pleaded guilty Aug. 31 to income-tax evasion and racketeering through prostitution charges, agreed under a plea bargain with federal prosecutors to pay the goverment $1.5 million from the sale of the entire parcel.

Antonelli reportedly planned to raze the property and turn it into an office complex after leases expired for the sexually oriented businesses it housed.