Two firefighters who were outside 2310 Naudain when it exploded were injured - one is still off duty. And PattiLynne Meadows, a condo resident who was evacuated before the blast, was hospitalized for two days with carbon monoxide poisoning.
The residents are still recovering from the shock. And answers to their persistent question have been exasperatingly elusive: What went wrong that night that upended their lives?
Initial suspicion focused on a natural gas leak. Investigators then turned to an underground fire in the electrical-distribution system that night, and whether it emitted explosive levels of gas that caused the blast.
Neither the city's gas utility, Philadelphia Gas Works, nor the electric company, Peco Energy Co., accepts responsibility. Their careful explanations increasingly seemed to be framed with a legal defense in mind.
The fire marshal's report was inconclusive. Investigators determined there was an electrical fire that started in front of 2300 Naudain, but they can't definitively link that smoldering cable to the explosion in 2310, though there is evidence the electrical fire reached a junction box inside the house that blew up, Deputy Chief Harry Bannan said.
"Neither the ignition source nor the fuel source for the explosion at 2310 Naudain Street could be determined," the department said in a May 30 release. It has not yet released its final report.
"Like many of you, I am disappointed that the investigation gives no definitive answer as to the cause of the explosion," Councilman Kenyatta Johnson told residents in a June 12 letter. "It is unsettling to think that the scientific tools at our disposal might not be precise enough to predict and prevent another catastrophe."
The same block of half-million-dollar rowhouses experienced an underground Peco electrical fire in 2005 - a different cable on the opposite side of the street. According to fire records, three houses suffered fire damage and virtually every house on the north side of the block suffered smoke damage.
"The first time it happened, we were assured we were all safe," said Judith Parker, who lives across from the May 1 blast. "Now this."
The residents see themselves as victims of the city's aging infrastructure, and regard officials and corporations as either incompetent or indifferent to the danger. In their state of heightened awareness, every transformer fire, every gas leak, and every water-main break is further proof of the decay.
"If it happens to us, it could happen to anyone," said Laura Brennan, a resident in the Naudain Court condominiums at the corner of 23d Street, where several of the 22 units were damaged or destroyed by the May 1 fire.
Many residents attribute the low injury toll May 1 to the alertness of a resident with a carbon monoxide detector.
Luke Prifogle, a 24-year-old architecture student, said he smelled a strange metallic odor about 2 a.m. in an upstairs unit he rented in Naudain Court.
He opened the window to let in some fresh air. The odor got worse. His CO detector shrieked.
His girlfriend called 911.
Firefighters responded quickly, and measured carbon monoxide levels at ground level exceeding 2,000 parts per million, the upper limit of their detection equipment.
Carbon monoxide is an odor-free gas that is emitted from incomplete combustion. Above 35 parts per million, it is poisonous. At high levels, it can kill.
At even higher concentrations - above 124,000 parts per million - it is explosive in the presence of an ignition source.
A PGW crew confirmed the Fire Department's readings. They ventured inside 2310 Naudain to take measurements a few minutes before the dwelling blew up and measured CO levels exceeding 2,000 parts per million.
But they also detected natural gas at about 1/20 the amount considered explosive, injecting a note of uncertainty into the equation.
The utility later measured low levels of natural gas in the soil beneath the sidewalk of 2310 Naudain, but said the gas did not match the chemical signature of the natural gas it delivers.
"We're confident our product was not involved," said Barry O'Sullivan, a PGW spokesman. It could be naturally occurring gas from decomposition of organic matter in the soil.
Nobody reported smelling natural gas that night. PGW also said it found no leaks in its gas main, which it replaced last week, partly to assure residents they are safe, but also to gather evidence for its defense.
The Pennsylvania Public Utility Commission is investigating the incident. The agency's gas safety division has decided it does not have jurisdiction over the matter, said Jennifer Kocher, a PUC spokeswoman. The PUC's electric safety division is conducting the investigation.
Peco admits its power lines caught fire. Spokeswoman Cathy Engel Menendez said torrential rains that night may have triggered the fault that caused the 240-volt cable to ignite.
It's not uncommon for burning plastic cable insulation to emit carbon monoxide. The New York State Department of Health in 2004 documented 234 events over 54 months linking CO releases to underground cable burnouts.
But Peco said that its expert maintains the Naudain Street fire could not have produced enough CO to explode.
"We do not believe the levels of carbon monoxide emitted from the fire in front of 2300 could have caused the explosion in 2310 Naudain Street," Engel Menendez said.