The sulfurous odor was detected across such a broad swath of Philadelphia's suburbs, and all from indoor locations, that Peco investigators suspect the source was not a leak, but an elevated level of the odorant mercaptan, which is added to natural gas to make it detectable.
Engel Menendez said the search for the stink had extended to the two interstate pipeline systems that deliver gas to Peco.
"That's how we're following the trail," she said.
Spokesmen for the pipelines said they were not at fault, and directed inquiries back to Peco.
"The odor is not coming from us," said Phil West, a spokesman from Spectra Energy, which operates the Texas Eastern Pipeline system. "We tested the line out. We're confident it's not from our system."
"All of our systems were operating normally," said Chris Stockton, a spokesman for Williams Cos., which operates the Transco Pipeline.
No other utilities that draw gas off the same interstate pipelines experienced similar problems. Philadelphia Gas Works did not get a whiff of customer complaints, said Barry O'Sullivan, PGW's spokesman.
Methyl mercaptan, also known as methanethiol or by its chemical formula CH3SH, is added to odorless natural gas to make a gas leak instantly recognizable to most humans.
Chevron Phillips Chemical Company L.P., which manufactures an extensive line of mercaptan products under the Scentinel brand, describes the odor as "repulsive."
Natural gas producers are required to add mercaptan to the product before sending it to market in transmission lines. Utilities such Peco can boost mercaptan levels before sending the gas on to customers via their local distribution lines.
One of Peco's central delivery points on the Transco Pipeline is in West Conshohocken, where gas is distributed to locations including Delaware and Montgomery Counties.
Mercaptan overloads are not unheard of. Three decades ago, Peco workers in Chester had to inject mercaptan manually because of a faulty pipeline, and they overdid it.
"A flood of customer complaints," primarily in South Philadelphia and Delaware County, sent "scores of utility work crews into a state of emergency readiness," according to a 1987 Inquirer report.
Customers who suspect a gas leak should call Peco at 1-800-841-4141.
Philly.com reporter Peter Mucha contributed to this article.