UPDATED 02/15/11 12:50 p.m.

ELGIN, Ill. (CBS) – Investigators have yet to determine the source of fumes sickened a number of people Monday night in an Elgin apartment building.

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Late reports indicated up to 12 people complaining of illness, along with two firefighters and a police officer who had been clearing the scene at 1131 Birch Dr. in Elgin.

The six-unit apartment building had been evacuated earlier in the evening because of concerns about unknown fumes and possibly dangerous chemicals.

But firefighters in hazmat suits searched the building, and about 8:30 p.m. reported that their detection devices gave no readings for anything dangerous.

As Elgin firefighters began to clear the scene sometime after 9 p.m., two firefighters and a police officer became ill and were taken to Sherman Hospital in Elgin, city officials said. One firefighter and a police officer complained of chest pains; another firefighter developed high blood pressure, officials said.

The six-flat apartment building was then red-tagged, and fire officials told the building owner he must contact an air-quality remediation company to determine the cause of the odor before anyone would be allowed back in, officials said.

The Elgin Fire Department also worked to determine if any of the families in the building needed Red Cross assistance.

About 9:45 p.m., calls went out indicating at least 12 patients complaining of illness.

Fire crews wearing hazmat suits entered the six-unit apartment building about 8 p.m. to investigate the source of the fumes. The odors reportedly were causing watery eyes and a metallic taste in the air.

Up to that point, there were no reports of anyone becoming ill or being transported to the hospital because of the fumes, authorities said.

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About 8:30 p.m., the fire crews left the building and said their search had turned up nothing that appeared to be hazardous.

As a precaution, though, the hazmat suit-wearing firefighters were scrubbed down in an outdoor shower.

Firefighters had been called to the scene about 5:20 p.m. when a resident in one of the units reported an odd odor. After firefighters arrived and began investigating, residents of another unit returned home and said they had smelled the odor about two hours earlier but did not report it and then left for a time.

Authorities eventually determined the odor was not natural gas and turned their investigation toward the possibility of hazardous substances.

Elgin Fire Department Incident Cmdr. Bryan McMahan said something was causing the odor — firefighters themselves could smell it — but their detection equipment picked up no readings of anything.

Authorities evacuated the two-story apartment structure but eventually allowed residents in neighboring buildings to walk to their homes. No one was allowed to drive in the area.

After finishing their search of the building, firefighters began ventilating the structure to remove remaining fumes and continued to investigate. It was not known when the residents of that building would be allowed to return to their apartments.

As of midday Tuesday, the cause of the noxious fumes remained undetermined, according to Elgin city spokeswoman Sue Olafson.

“The source has been unidentified at this point. I think that the hope is that once the building owner contacts this air quality remediation company, that they would be able to determine the cause,” Olafson said.

Elgin firefighters were assisted at the scene by crews from South Elgin, Streamwood, Carpentersville and Hanover Park, and by the Kane County Office of Emergency Management.

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The Sun-Times Media Wire contributed to this report.