Unified Fire Authority (UFA) crews evacuated all residents of a six-story Millcreek apartment building Monday, Oct. 5, after carbon monoxide readings registered dangerously high throughout the complex.

A CO alarm sounded on an upper floor of the Glenwood Apartments at 3993 S Main St. around 2:35 p.m., according to SLCScanner, which monitors emergency dispatches. Readings were high enough that the entire building was cleared. Main Street was closed in both directions while crews worked the scene.

No one was injured.

Hazmat teams entering the building needed masks to work safely inside, UFA Capt. Matthew McFarland told Fox 13. "[Hazmat teams] were getting positive readings, far above healthy levels," McFarland said. "To the point, we were working on our masks on the inside."

The building holds more than 165 units. UFA crews ventilated the structure while investigators searched for the source of the gas, according to ABC4.

One resident, Dawn Alonso, told Fox 13 she had been drinking coffee while her son slept when the alarm forced everyone outside. Alonso said a friend died from carbon monoxide exposure in a different building about six weeks earlier.

Residents were allowed back into the building later Monday evening after it was deemed safe, according to Fox 13. KUTV reported that as of Monday evening, crews had not yet identified what caused the elevated readings.

UFA has not publicly announced the source of the carbon monoxide. McFarland urged residents to keep CO monitors at home alongside smoke detectors and to change the batteries twice a year.

Millcreek residents who suspect a carbon monoxide leak should call 911 and leave the building immediately.