O'HARA TOWNSHIP, Pa. -- Four people from O'Hara Township were treated at a hospital for symptoms of carbon monoxide poisoning after being exposed to "extremely high levels" when someone apparently left a vehicle running inside a garage.

Allegheny County emergency officials told Channel 4 Action News that a carbon monoxide detector started going off at The Docks condominium complex on Riverfront Drive just after 2 a.m. Tuesday.

Kristin Demkoski, a tenant, said police woke her and told her that "we needed to evacuate right away because there are dangerously high levels of carbon monoxide in the building."

Close to 30 people were evacuated and emergency crews discovered high levels of carbon monoxide in one of the third-floor apartments.

Equitable Gas said the CO measured at 700 parts per million -- much higher than 35 ppm, which the Allegheny County Health Department said is the maximum allowable level for eight hours.

"Those are extremely high levels. Anything from 400 to 800 parts per million can result in death in two to three hours," said Guillermo Cole, a spokesman for the Health Department.

Channel 4 Action News' Amber Nicotra reported that fans were used to clear the air in the building.

When emergency crews arrived at the scene, they said they found a utility truck that had been left running in a garage and may have been the source of the carbon monoxide, according to police.

Residents were allowed to return to their homes around 5 a.m.

No charges have been filed. Police are still investigating.

Nationwide Realty -- which owns The Docks -- has ordered Golon Construction to stop work at the site immediately.

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What is CO?

Carbon monoxide is a colorless, odorless gas that can cause brain damage and death. Symptoms of carbon monoxide poisoning include headache, nausea and fatigue.

Of the four people being treated, two were taken to UPMC Presbyterian Hospital where one of the victims is listed in serious condition.

"It's a real good thing that someone had a detector. That's why it's important for people to have them -- to catch the gas before it reaches a lethal level," Cole said.