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SKIPPACK, Pa. (AP) — Residents of about 150 homes in suburban Philadelphia began returning to their neighborhood Monday after a mysterious odor prompted a voluntary evacuation.

Environmental officials are awaiting lab results to identify the substance responsible for Sunday evening's problem in Skippack. Authorities said the smell originated in basement sump pumps, but they have not been able to pinpoint the cause or composition of the chemical odor.

Fire Chief Haydn Marriott 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.

Air and water samples have been sent to four laboratories in an effort to detect the compound, which officials only know is "some kind of hydrocarbon," Marriott said. He hoped to get results back early this week.

"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."

He noted the substance was found in a system with sealed pipes creating little chance of contamination to the surrounding area's water supply.

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

Sensors originally indicated the presence of hydrogen cyanide, a poisonous gas, in the homes, but officials later said they believe those readings were false positives.

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 the townhouse development for 10 years and never had a problem with sump pumps.

"It's still really bad in our house," she said of the odor late Monday morning. Her home had not yet been cleared for her family to return, she said.

Early Monday afternoon, officials began investigating a retention basin behind Loughin's house. The basin has occasionally flooded into her home during storms, Loughin said, but never accompanied by a noxious odor.

George Berry's home across the street had 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 officials took a reading in their home Sunday evening and advised them to leave. "We kind of want to see what that next test is."