Authorities are continuing to investigate a home that exploded in Center City overnight, causing a three-alarm blaze that spread to other residences.

It remains unknown what caused explosion and fire, which displaced dozens of people on the 2300 block of Naudain Street, city officials said early this afternoon.

The explosion did not appear to cause any injuries, though one woman had been hospitalized earlier after inhaling carbon monoxide.

Ten homes were damaged, six severely, fire department officials said this afternoon. The other four sustained mostly water damage.

It may be two days before the 40 displaced residents can return to their homes, Mayor Michael Nutter said.

Around 3 a.m., some residents noticed a gas smell and called 911. Firefighters detected high carbon monoxide readings throughout the entire block. Residents were evacuated as emergency crews attempted to locate the source.

But something ignited an explosion at one home, causing the blaze, authorities said. The fire was brought under control shortly after 5 a.m.

Though residents reported a gas-like odor, it's still too early to say what sparked the explosion and fire, Nutter said.

"We do not know that there was a gas explosion," he said.

Brian Moores, who has lived on the block for 20 years, said the carbon monoxide alarm for one of the building's residents sounded about 3 a.m.

The fire department responded, checked CO levels on the block, and found high readings. One woman in the building was overcome by the carbon monoxide, Moores said, and was taken to a hospital. The woman's cats died. Crews were dispatched to help locate the source of the carbon monoxide.

Then, at about 3:30 a.m., a home on the block exploded, Moores said.

The house was "blown into pieces," said Moores, who lives across the street. Debris from the explosion spilled onto the sidewalk and street.

That home caught fire, which spread to three adjacent houses.

Fire crews were already on the scene, but it took about 15 minutes to get water running to douse the blaze, Moores said. The home appeared to be destroyed.

No one was hurt in the explosion, as residents had already been evacuated while authorities probed the suspected gas leak.

Moores said the explosion didn't damage his house across the street, but he didn't know yet if the building sustained smoke or CO damage.

Officials from the Department of Licenses and Inspections are surveying homes on the block.

Staff from the American Red Cross of Southeastern Pennsylvania -- already racing about all night from helping with flooding -- set up a shelter for displaced residents at the High School for the Creative and Performing Arts on South Broad Street.

At least 30 people have gone to the shelter since early this morning, the Red Cross said, and nine people remained there at mid-afternoon.