Updated

Residents of 150 suburban Philadelphia homes are being allowed to return home as their houses are tested after a voluntary evacuation overnight due to a mysterious odor.

Officials are awaiting lab results to try to identify the substance responsible for Sunday evening's problem in the neighborhood in Skippack. Officials have identified basement sump pumps as the origin but have not been able to pinpoint the cause.

Authorities say they believe a type of organic substance emitted as gases from thousands of products was responsible. The products include paints, lacquers, cleaning supplies and stored fuels. But officials say there have been no signs of illness.

Fire Chief Haydn Marriott, however, told reporters Monday that there were "absolutely, positively no signs of illness." Some people went to hospitals Sunday night to be checked out as a precaution.

An emergency shelter was set up at an elementary school, and the Red Cross said 12 people from four families were put up for the night.

Marriott said both air and water samples had been sent to four laboratories to try to identify he substance, which officials only know is "some kind of hydrocarbon," and hoped to get results back in the afternoon.

"The highest readings are in the sump pump pits, and we're only getting the readings in the pits that have water in them," Marriott said. "It is very apparent that it's traveling somehow in that system with the water, but it's hard to tell how and why at this point."

But, Marriott said the substance was found in a system with sealed pipes, so there was little chance of the surrounding area's water supply being contaminated.

"We did speak to the water authority several times yesterday; they assured us that there was no problem with the water, that they were going to take care of the testing on their end," he said Monday.

Officials originally thought the material was hydrogen cyanide but now believe the compound is mimicking whatever registers as that substance on the sensors.

Authorities had tested about half of the homes by noontime Monday and said they were safe for residents to return with little or no evidence of contamination.

Kourtnay Loughin said she was home Sunday night when she noticed a "very, very unpleasant" odor and called 911, waiting outside with her husband and three children. She said they have lived in home for 10 years and never had any problem with the sump pumps.

"It's still really bad in our house," which hadn't yet been cleared for her family to return, she said.

George Berry's home across the street has been cleared, but he and his wife planned to keep their children with relatives until the evening.

"We're still a little nervous," said Berry, who said they smelled nothing before being advised to leave. "We kind of want to see what that next test is."