Thirteen firefighters were injured and nine families were left homeless yesterday in a fire in a three-story Dale City apartment building that fire officials called the worst blaze in Prince William County in several years.

More than 125 firefighters battled the four-alarm blaze at the Bayview Apartments in the 1300 block of Bayside Avenue for almost an hour before bringing it under control, said county fire spokeswoman Dottie St. John. Damage was estimated at $500,000.

Residents were notified by the Fire Marshal's Office that they would not be able to return to the building because it is unsafe. The roof was destroyed, as was most of the third floor. The first and second floors were reported heavily damaged from water and smoke.

No residents were injured. Only about a half-dozen people were at home when the fire broke out about 1:55 p.m.

The firefighters' injuries included heat exhaustion, smoke inhalation, minor burns and lacerations. The firefighters were treated at Potomac Hospital.

Yesterday's 98-degree heat combined with the firefighters' heavy uniforms was conducive to heat exhaustion, St. John said.

The uniforms, designed to keep out heat and resist cuts, also hold in heat, cutting down on the length of time the firefighters can work near flames before breaking away to cool off.

"The combination of the firefighting gear and the heat and humidity made it impossible for them to stay in there very long," St. John said.

A woman who identified herself as the apartment manager said the building was 21 years old and that there had been no reports of faulty electrical wiring or other problems that might have led to fire. She said the fire started in the attic.

Fire officials were investigating the cause of the blaze.

The woman said the Red Cross was helping the displaced families in finding shelter in local hotels. It was not immediately known how long the building will be uninhabitable.

Another building in the apartment complex was damaged in a similar blaze eight years ago, St. John said.