AUGUSTA — Investigators say the fire that destroyed an Augusta apartment house Monday was caused by a 5-year-old playing with either matches or a lighter.

Three families are homeless in the wake of the blaze at 94 Mount Vernon Ave., which shut down traffic on the busy street during afternoon commuting hours. Neighbors and passers-by watched as more than two dozen firefighters worked to knock down the intense fire.

Steve McCausland, spokesman for the Maine Department of Public Safety, said in a statement Tuesday that the fire started in the first-floor apartment and quickly spread throughout the house.

McCausland said no one was injured.

Augusta Deputy Fire Chief Dave Groder said it’s not clear whether a match or a lighter was used.

On Tuesday, the burned building stood empty on the street, with bits and pieces of broken glass and the tenants’ belongings on the ground, including a bright red fire extinguisher that apparently never was used. Plywood covered the front entrances and utility wires hung slack from the building’s north side. On the south side, three white propane tanks, one for each unit, stood disconnected and slightly askew.

A woman stopped by to take a look around. She said she had lived in the building and had lost a dog and four cats, but otherwise she declined to be interviewed.

The building won’t be standing for much longer. Steps are underway to demolish the structure because it is next to the road.

Rob Overton, code enforcement officer for the city of Augusta, said he had advised the building’s owner on Monday to have the building demolished as soon as possible, once it has been cleared for that by the Office of State Fire Marshal.

“I don’t believe we’ll require a permit,” Overton said. “It’s a safety issue. We’ll waive the fee to expedite the process.”

Augusta property records show the building is owned by Tomberlin Construction. George Tomberlin said demolition could take place as soon as Wednesday, but certainly by the end of the week.

Anne Kim, spokeswoman for the American Red Cross, said those affected by the fire would be offered financial assistance.