Fire crews continue to investigate the cause of a massive fire that tore through a northwest Calgary strip mall, destroying four businesses and severely damaging two others.

At about 7 a.m., crews were called to the Stadium Shopping Centre, located at 1941 Uxbridge Drive N.W., after a passerby reported seeing smoke pouring from the top of one of the businesses in the mall.

Fire crews called a second alarm due to the volume of smoke and fought aggressively to contain the fire, but the battle lasted for several hours as the blaze kept on flaring up in the structure.

"The issues we're running into is the roofing material itself, which is a combination of drywall, straw, tar and metal and the fire has gotten in between the layers of the straw and has started to move around the building itself," said CFD Battalion Chief Al Magwood on Thursday.

On Friday, a small fire flared up among the debris of the building as crews were working to clear it up and fire crews were at the scene to keep control of it.

Business owners, who were all struggling with the fact the building, built in the 1960s, was slated for demolition next year, are devastated about the fact they've lost their businesses earlier than expected.

The Calgary Board of Education had closed two schools because of poor air quality caused by the fire, and say they are keeping University Elementary School closed on Friday because of those same issues.

Currently, the air quality in the area is about a three out of 10, which is a low risk, but there still is a strong scent of smoke in the air.

The air is expected to improve over the weekend so the school can reopen for Monday's classes.

As for the businesses in the Stadium Shopping Centre, many owners say they will be speeding up their work to secure a new location.

The developer in charge of the new site says they are reviewing the extent of the damage and working with tenants to determine the appropriate solution.