Two people are dead, one person is critically injured and four first responders have been hospitalized in a mysterious hazardous materials situation in unincorporated Des Plaines.

Around 3:30 p.m., police were called to a northwest suburban townhome in the 9400 block of Harrison Street after receiving a report of three people in distress.

Upon entering the home, four Cook County sheriff's officers were unexpectedly overcome by some sort of hazardous agent, authorities said.

“They started feeling sick,” said Rick Dobrowski, chief of North Maine Fire Protection District. “They were having some kind of breathing difficulties, we don’t know for sure.”

The officers were taken to area hospitals to be treated. They have since been released and are in stable condition.

Inside the home three adults – two males and a female – were found not breathing, according to police.

The Cook County medical examiner pronounced both men dead and the woman was transported to a hospital in critical condition.

The two men were identified Friday morning as 53-year-old Hong K. Kim and 83-year-old John Tae Kim, according to the medical examiner's office.

A preliminary investigation revealed the victims likely died due to inhalation of a deadly substance.

No information has been released as to what room of the house they were in or their relationship with one another.

Several residences around the home were temporarily evacuated while hazmat teams worked late into the night Thursday to try to figure out what that substance is.

“They didn’t find anything in particular but they were able to rule out anything that was bad – as far as any kind of alkali or acid that was still in there, fumes” Dobrowski said. “They were also able to test the water that they found to make sure there was nothing mixed, chemical wise.

Police said surrounding residents are not in danger, but that doesn’t mean they weren’t on edge as they surveyed the scene of dozens of emergency crews.

“I’m terrified,” one neighbor told NBC Chicago.

Investigators remained at the home near Potter Road well after dark to continue to sweep for evidence.

“They’re just trying to determine what possibly caused this,” said Cook County Sheriff's spokeswoman Sophia Ansari.