Investigators with the State Fire Marshall's office and the Bureau of Alcohol, Tobacco and Firearms wait on Monday for a backhoe to spread debris that was pulled out from inside the first floor of 25 Greene St. in Biddeford. Officials have not determined if the building is safe to enter, so they are beginning their investigation by examining debris pulled out by the backhoe.

Investigators with the Bureau of Alcohol, Tobacco and Firearms, the State Fire Marshall's Office and the Biddeford Fire Department examine the inside of the ground floor of 25 State St. in Biddeford on Monday, looking for clues to what caused the fire in the building on Sunday night. Residents of the apartment building say they believe the fire started in the back of the building.

A Biddeford firefighter climbs down a ladder after dousing some hot spots at 25 Green St. in Biddeford on Monday. Officials have not yet determined if the building is safe to enter and have not been able to start the investigation into the cause of the fire.

Biddeford firefighters are still cleaning up this morning at 25 Green Street in Biddeford, the scene of a large fire on Sunday night that displaced about 20 people. Gregory Rec/Staff Photographer

Dennis Lund, Jr., who lives in a third floor apartment at 25 Green St. in Biddeford, talks to the media outside the apartment building on Monday. Lund was alerted to the fire by someone knocking on his door and he was able to escape with his two cats.

A ladder truck was still putting water through the fallen roof of 25 Green Street in Biddeford, the scene of a large fire on Sunday night that displaced about 20 people. Gregory Rec/Staff Photographer

A Biddeford firefighter lowers a hose on Monday morning from the back porch of 25 Green Street in Biddeford, the scene of a large fire on Sunday night that displaced about 20 people. Gregory Rec/Staff Photographer

A firefighter walks up a ladder and into a building on Greene Street in Biddeford, on Sunday Whitney Hayward/Staff Photographer

Firefighters battle a fire at an apartment building on Green St. in Biddeford Sunday. Shawn Patrick Ouellette/Staff Photographer

Firefighters can be scene battling a Green St. apartment fire from Crescent St. in Biddeford SundayShawn Patrick Ouellette/Staff Photographer

Neighbors watch as 25 Greene Street, the building across the street, burns while firefighters try to put out the flames in Biddeford, ME on Sunday. Whitney Hayward/Staff Photographer

Firefighters can be seen through the smoke as they battle a fire on Green St. in Biddeford Sunday Shawn Patrick Ouellette/Staff Photographer

A firefighter unrolls a hose from an engine to put out a fire that broke out at 25 Greene Street in in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

Fire at 25 Greene Street in Biddeford, on Sunday, Whitney Hayward/Staff Photographer

Firefighters spray water on the roof of 25 Greene Street in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

A firefighter watches the fire at 25 Greene Street as smoke billows in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

Firefighters extinguish a blaze at 25 Greene Street in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

Celina Nolette, a neighbor from across the street from 25 Greene Street, puts her shirt over her nose as smoke billows in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

Firefighters spray water on flames bursting through a second story window at 25 Greene Street in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

Flames come through windows at 25 Greene Street in Biddeford, on Sunday.Whitney Hayward/Staff Photographer

Onlookers watch as firefighters battle a fire at a downtown apartment building on Green St. in Biddeford Sunday. Shawn Patrick Ouellette/Staff Photographer

Biddeford Fire Chief Joseph Warren talks with media at the scene of a fire on Green St. in Biddeford Sunday. Shawn Patrick Ouellette/Staff Photographer

Firefighters gather and prepare to fight a fire at 25 Greene Street in Biddeford, on Sunday. Whitney Hayward/Staff Photographer

Water is sprayed on parts of the building at 25 Greene Street, to prevent flames from burning the front of the building in Biddeford, on Sunday. Whitney Hayward/Staff Photographer