Investigators are trying to figure out what caused a gas-fueled explosion at a three-story garden-style apartment complex in Northeast early Wednesday morning.

Neighbors say they heard a boom and then saw the explosion about 1:25 a.m. in the 4300 block of Dix Street, NE.

"I had my dogs out front, and I went to let them in and by the time I shut my door I heard a large... a loud boom - it sounded like a bomb went off," said witness Ammeta Smoot.

None of the three people inside the building at the time of the explosion was hurt.

The explosion blew out windows and doors. One window frame landed on a parked car. Other debris hit overhead utility lines.

"It looked like the door had been blown off the building," recalled Smoot. "I seen the wood and everything across the street, so I didn't know exactly what had happened at the time."

Washington Gas crews shut off the gas to the building and conducted an investigation. They report that no leak was found around the outside of the building.

ABC 7/NewsChannel 8 sources say police have found evidence that someone inside the building had been working with the copper pipes and that may have caused the explosion.

D.C. Fire and EMS has cordoned off the building until structural engineers determine whether it is safe.

"We set up a collapse zone. We're not going to let anybody in the collapse zone for fear that parts of this building might come down," said D.C. Fire and EMS spokesperson Alan Etter.

The investigation remains on-going.