The gas reportedly killed a University of Denver graduate student and sickened a second student Monday night at the Josephine Place Apartments, the Rocky Mountain News reported Tuesday.

The gas leaked from a flue on a boiler unit that had been repaired last week after strong winds damaged it, the News reported.

The apartment contained carbon monoxide levels as high as 1,500 parts per million when levels of 1,200 parts per million are considered deadly, said Denver fire officials.

An estimated nine people on average die in Colorado each year from the odorless and colorless gas, the News reported.