Agents from the Bureau of Alcohol, Tobacco and Firearms are working alongside San Francisco fire investigators to figure out what happened. They're sifting through the debris in the 100 block of Congo Street trying to figure out if the explosion was accidental or intentional.
Federal investigators arrived at what remains of the house on Congo Street and were soon joined by the San Francisco police department bomb squad. All were called in to figure out what caused this destruction at about 7:45 Monday night.
"We need some assistance in determining cause and origin, so anything unusual like you see here this morning, we will ask for assistance from the ATF," said Chief Joanne Hayes-White, San Francisco fire department.
By midday, investigators had ruled out the most obvious causes of the blast, including natural gas and electricity.
The blast sent out wood and glass shooting across the street and into the homes nearby. It blew out the back window of a car parked across the street. A two-year child in a neighboring house suffered facial injuries when a nearby window shattered. At least ten people in five homes were evacuated. All have been allowed back in except for one.
Neighbors described the blast.
"I can't explain it, I've never heard it before, a really huge, huge explosion," said neibgor Cesar Gonzales.
"I heard a big boom, it felt like something was running into the house actually," said another neigbor, Margie Cleland.
Four people were inside the building at the time of the blast and all were treated. Two were burned, another injured his back, and the fourth is an elderly woman who was taken in for observation.
The home is owned by Barbara Riccardi, a well-known violinist who plays for the San Francisco Opera Ochestra. Ofa Fifita is the caretaker of Riccardi's 97-year old mother. She lives in the rear of the house which was not damaged. "I was in the middle of feeding her. She has two feedings. And I heard something like its gonna collapse," said Fifita. Noma Otuafi also lives in the house with her aunt Ofa. Noma wasn't home Monday night, but she has noticed some guys hanging out in the downstairs area where the explosion happened.
"I know the boys who live there, there are a lot of them lately - and they all stay in there," said Otuafi.
Investigators were looking at the possibility they may have accidentally caused the explosion. Firefighters had to shore up the house with a post before the police bomb squad and ATF investigators could enter. Among the evidence taken from the home were two pieces of pipe and a bicycle which a police source says was attached to a chainsaw motor. Police say they've talked to the three men in the hospital and declined to say much else.
"They have spoken to our investigators, but at this time, its part of the investigation," said Sgt. Wilfred Williams, San Francisco Police Department.
A van that belongs to the men who live in the house has been towed from the area. The bomb squad has found no evidence of any homemade explosive devices, but an investigation continues.