A mysterious smell forced firefighters to evacuate more than 100 homes in a Philadelphia-area community on Sunday, CNN reports.

Residents are being given the green light to return home Monday as officials work to identify the cause of the strange smell, The Associated Press adds.

Officials believe that basement sump pumps are the origin of the smell but they have not been able to determine the cause.

The Skippack Township Fire Company said in a statement that air and water samples have been taken by the U.S. Environmental Protection Agency to identify the nature of the Volatile Organic Compound.

According to the Environmental Protection Agency, VOCs “are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. Concentrations of many VOCs are consistently higher indoors than outdoors.”

The EPA says that examples of VOCs include paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions.

The fire company noted that none of the evacuated residents showed any signs of symptoms of exposure to a chemical.

There were “absolutely, positively no signs of illness,” Fire Chief Haydn Marriott told reporters on Monday, according to The AP.

The Skippack Fire Company, Montgomery County Hazardous Materials Response Team, Pennsylvania Department of Environmental Protection and the federal Environmental Protection Agency are working to determine the VOC involved.

The Fire Company said that the Red Cross opened a shelter at a nearby elementary school for evacuated residents.