The troubles began Sunday about 5:45 p.m., after a North Gorski Lane household reported smelling a strong odor.
An initial meter reading by emergency responders incorrectly showed the presence of the potentially harmful gas hydrogen cyanide, said Skippack Township Fire Company Chief Haydn Marriott.
Notes put on residents' doors said a "volatile organic compound" - which can be found in household products - had been detected.
So, 151 homes were evacuated.
On Monday, the mystery was solved. Tests showed that gasoline in sump pump pits in three houses caused the odor that sent the neighborhood into an uproar.
"No idea how it got in there," said Marriott, who added that no one had reported health problems.
As word spread Sunday, residents scattered with some spending the night with family and friends, and at hotels. An emergency shelter at a nearby elementary school and a medical tent were also set up.
Jack Hughes, 64, who lives next to the Berrys, moved to the neighborhood just six weeks ago with his wife and two sons, 20 and 22.
The family said they like the neighborhood. It is friendly - even during a crisis.
As households were evacuated, some "tailgated it," drinking beer in the street as they waited for word from authorities, said Hughes' son, Michael, 22.
"We had a hydrogen cyanide party," he said with a laugh, referring to the initial reports of that compound's presence.
Residents returned to their neighborhood Monday morning to chat with each other and watch emergency crews and news vans pack their usually-quiet development.
Emergency officials, stationed at a command post established in the neighborhood, would not let them back into their homes until a monitoring team had tested levels of various chemical compounds to make sure the conditions were safe.
Late Monday, most of the residents were able to return, and officials still were flushing out the contaminated sump pump pits and ventilating three homes, Marriott said.
Emergency personnel - local, state and federal authorities were helping with the investigation - planned to keep monitoring the water and air. Marriott warned that the odor could come again.
And the Berry family?
Still worried about where the contamination had come from, they planned to spend Monday night with George Berry's parents. That was fine with the girls.
"They're with their grandmother," Christina Berry said. "They're being spoiled."