The mystery substance was seen on resident's roofs, cars and porches

A city in Michigan is perplexed after a a tar-like substance has rained down on their cars, porches and driveways this week.

The black, oily substance first appeared on at least six driveways in Harrison Township on Sunday, and days later, what the material is still remains a mystery.

Michigan Department of Environmental Quality officials collected samples of the substance on Wednesday.

A black, oily substance first appeared on at least six driveways in Harrison Township, Michigan, on Sunday

The mysterious substance remained on outdoor surfaces through Wednesday, when Michigan Department of Environmental Quality officials collected samples

The city's fire chief sad that it is not bird droppings and is not flammable, according to WXYZ.

Harrison Township resident Paul Schlutow, 73, said 'everybody's concerned' about the substance and the major concern is that the substance could potentially be harmful.

Residents originally believed that the substance could have come from the nearby Selfridge Air National Guard Base, but the base released a statement saying it was not coming from their location.

'There is no indication that the substance in question came from a military aircraft of any type,' the statement said.

The statement said the airbase has 'been in communication with the Michigan Dept. of Environmental Quality, which was sending a representative to the area in question to review the material'.

Residents originally believed that the substance could have come from the nearby Selfridge Air National Guard Base, but the airbase released a statement saying it was not coming from their area

Residents of the neighborhood have been trying to clean off their cars, porches and driveways since the substance arrived

Residents said they are concerned the substance could be toxic. Officials assured them it was not flammable

'As Michigan's Hometown Air Force, we take being a good neighbor very serious,' Brig Gen John D Slocum, commander of the 127th Wing and the Selfridge base commander told ABC News. 'We will continue to work with our local and state partner agencies to resolve this question.'

Fire Chief Michael Lopez told Fox 2 Detroit that the airbase will still investigate.

'It does not appear to be a fuel substance at all and so we did confirm that but it appears as though it came from an aircraft but we haven't confirmed that but we did turn it over to Selfridge to investigate,' he said.

A Michigan Department of Environmental Quality officer said it could be at least a week before the organization discovers what the substance is.