Overnight, a small town in Michigan was blanketed with a layer of viscous black goo. It came down in 26,000-person Harrison Township earlier this week, causing alarm and a slew of questions, foremost among those being: What is this stuff?

According to reports, the tar-like substance came raining down from the sky all over the area, with one resident telling The Macomb Daily that at first, he “thought it was bird crap.” WXYZ-TV News in Detroit reports that residents are angry about the ubiquitous substance:

What’s more, no one seems to know what the substance is, or where it came from. Fire Department Chief Michael Lopez said that the substance is not flammable, and “does not appear to be a fuel substance.” Chris Ethridge, of the Michigan Department of Environmental Quality (DEQ), told Grist that inspectors went out to Harrison Township on Wednesday to take a sample of the fallout, and won’t know its ingredients until a microscopic analysis is complete — and that could take weeks.

“This is the first time we’ve ever heard this kind of complaint in that area,” Ethridge said.

But there is one major suspect: nearby Selfridge Air National Guard Base. Many residents immediately speculated that the base, which sends planes screaming above Harrison Township, was to blame.

After residents called the base to complain, however, Selfridge denied any culpability.

“Based on all of these reviews, there is no indication that the substance in question came from a military aircraft of any type,” officials said, according to The Detroit News.

Ethridge confirmed that the DEQ “has suspicions that it’s coming from the base next door,” but added that he couldn’t confirm it just yet. Until the tests come back, it falls to residents to clean up the oily goop — without even knowing what it is yet.