Updated at 12:15 p.m. on Monday, Dec. 12

An incident that was originally reported as a natural gas leak in downtown Anchorage is now being investigated by the Anchorage Fire Department as carbon monoxide that wasn’t correctly venting.

Lindsay Hobson with Enstar said the company was called to the scene, but it turned out not to be an Enstar-related issue. She said it appears an appliance in the building was not working properly.

Capt. Josh Walker with AFD said it could have been caused by a vent that had frozen over.

“Keep those vents in your garage and other areas where your boilers are at clear and open,” he said. “Don’t plug them up. A lot of people tend to plug them up, and then that creates those same kinds of problems inside your house.”

A text message alert sent by the department on Monday morning stated AFD’s hazardous materials team was on the scene of a natural gas leak in downtown. Later, AFD emailed a release stating that at 10:04 a.m. a 911 call reported the gas leak, which originated in the boiler room of Alaska Cleaners on Fifth Avenue.

The department was on the scene one minute later, according to the release, when employees were evacuating the building.

Capt. Josh Walker with AFD told KTVA on the scene that multiple people were transported to the hospital for related injuries. The release stated that five people went to the hospital — four by ambulance and one in their personal vehicle. Several people were in serious condition, according to AFD.

Fire personnel secured the area and the boiler. The Anchorage Police Department was on scene to help control traffic as Cordova Street was closed to through traffic.