A powerful mysterious odor from an unidentified gas prompted a voluntary evacuation of about 150 homes in a Philadelphia-area community early Sunday evening after it was discovered to be leaking from the sump pumps in the basements, a fire department said. About 100 residents, however, were confirmed to be returning to their Montgomery County homes Monday morning after their basements' were metered.

After a resident complained of a strong odor in the Skippack Township neighborhood at about 5:45 p.m. Sunday, firefighters began evacuating homes, the Associated Press reported. More houses were later found to have levels of the organic compound as well when the township fire company responded with testing meters that "went into alarm" in their basements.

The Montgomery County hazardous response team, state Department of Environmental Protection and federal Environmental Protection Agency are working to identify the "volatile organic compound" involved, the county Department of Public Safety said in a statement Monday. Skippack Fire Chief Haydn Marriot made the announcement that displaced residents were allowed to return home after their home was metered. "We're checking each house individually and if the meter reads zero, residents are free to go back in."

According to the Environmental Protection Agency, volatile organic compounds cover a wide range of gases that can come from thousands of products, such as paints and lacquers, paint strippers, cleaning supplies, pesticides and permanent markers, CNN reported. They can find their way into ground water from spills or leaking storage tanks or in stormwater runoff from roads and parking lots.

Marriott had earlier told WPVI-TV in Philadelphia that although crews testing the homes had gotten high readings of a poisonous gas called hydrogen cyanide, a bi-product of fires that can be deadly in high concentrations, crews didn't actually think the odor in the homes was from hydrogen cyanide, but some other chemical. They didn't immediately confirm what the gas was.

Meanwhile, the American Red Cross of southeastern Pennsylvania set up an emergency shelter at the local elementary school to assist evacuated residents Sunday night. Some residents went to a hospital as a precaution and were released.

Residents are being asked to contact the township and have their homes tested for the presence of the compound before returning home. Additionally, samples of the compound have been taken to the Environmental Protection Agency, with results being expected on Monday afternoon.