It was hours after the explosion on Naudain Street when Christine Becer finally got a look at her home.

A Philadelphia Gas Works employee walked her across South 23d Street, where caution tape was still draped between light poles.

What she saw was almost too much to take in: a collapsing roof, a sagging window frame, and no clues as to what might await her inside.

The house next door was leveled. Bulldozers had already arrived to begin clearing the debris.

"I don't know what to say," the Center City lawyer said Thursday afternoon. "It's crazy - the idea of replacing everything."

The predawn blast had destroyed 2310 Naudain St., a rowhouse in the middle of a quiet Fitler Square street, and seriously damaged another house and four apartments in Becer's condominium complex.

About 40 people were evacuated from their homes, and 17 checked in to a temporary Red Cross shelter at the High School of Creative and Performing Arts on South Broad Street.

Firefighters had detected dangerously high levels of carbon monoxide on the block before the explosion and started evacuating homes.

One woman - who lived next door to the leveled house - was taken to the hospital suffering from respiratory distress, and her two cats died, Deputy Chief Robert Wilkins said.

Firefighters arrived on the street around 2:14 a.m. after an automatic carbon monoxide alarm sounded at 2310. They called for a battalion chief to check the levels of the odorless gas.

Those numbers came back dangerously high, Wilkins said.

"That concentration of fumes and oxygen - it's an ignitable source of fuel waiting for the right combination of air and heat," he said.

At 4 a.m., a battalion chief called in a second alarm, Wilkins said. As evacuations began, firefighters had looked into the home at 2310 and seen flames, Wilkins said.

Then, he said, the house exploded.

Neighbors said the residents - a woman and a young boy - made it out with their pets before the blast. The woman had lived on the block for decades, they said.

Firefighters called a third alarm about 4:20 a.m., Wilkins said.

The cause of the explosion and the high carbon monoxide levels are under investigation.

An official with the Philadelphia Gas Works said Thursday afternoon that it did not appear there was a problem with natural gas in the area.

Thursday afternoon, firefighters were still putting out hot spots at 2310 Naudain, and acrid smoke filled the air hours after the blast. The Red Cross said two cats and two dogs were rescued and were being cared for by Red Paws.

Mayor Nutter stopped by the shelter Thursday afternoon, his staff said, and spoke with the woman who lived at 2310, identified on property records as Joann Baskerville.

By Thursday evening, everyone at the shelter had found a place to stay.

On the corner, neighbors told of being awakened by firefighters pounding on their doors and hurriedly evacuating their homes.

Katherine Antarisko and her partner, Kamil Dmowski, were inside their condo when a firefighter told them to get out. Then they heard a boom, and the doors shook. They looked outside to see black smoke pouring from a house down the block, they said.

Betsy Alexander said she and her husband, fellow artist Burnell Yow!, had watched from a third-floor window across the street as Naudain filled with fire trucks.

The explosion shook the block, and "within seconds," she said, firefighters "were banging on our door. And they said, 'Get out.' "

Alexander and her husband were able to gather their seven cats and spent the rest of the day at a neighbor's house. Her home was undamaged, she said, but smelled strongly of smoke.

She described Baskerville as a 40-year resident with a well-kept garden. Neighbors would see the boy who lived with her skateboarding on Naudain, sometimes pulled by his dog.

"It's very, very sad for our neighbors," Alexander said.

She said the block had been shaken by the incident.

"We don't have a carbon monoxide detector in our house," she said, "but we're going to go get one."