(CNN) -

A strong odor from an unidentified gas led firefighters to evacuate 151 homes in a Philadelphia-area community, a fire department said.

Firefighters began evacuating homes from part of Skippack Township after a resident complained of an odor early Sunday evening, the township's fire department said.

With the help of meters, firefighters learned a gaseous volatile organic compound was coming from the basement sump pumps in several homes, the department said.

They didn't immediately know what the gas was. Samples were taken to two labs for tests, and results are expected Monday afternoon, Skippack Township Fire Chief Haydn Marriott said.

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, according to the Environmental Protection Agency.

The Red Cross opened a shelter at an elementary school for evacuated residents who needed a place to stay, the fire department said.

Firefighters were testing homes' gas levels again Monday, and evacuated residents were being allowed to return to homes where levels were low, Marriott said.

Skippack Township is about 25 miles northwest of Philadelphia.