Environmental experts spent Monday testing and re-testing air levels in a Boulder apartment that caught fire earlier this month, and fire officials said it could be a while yet before tenants are allowed back into the asbestos-contaminated building.

Boulder fire marshal Dave Lowrey said the 50-plus residents displaced from 30 units in the Hill House Apartments at 949 Marine St. could learn more about when they'll be allowed to return after results of new air samples come back.

"We will make a determination as soon as we know how bad it is," Lowrey said.

The four-story apartment building burned for hours July 12, scattering residents, most of whom were University of Colorado students. The blaze, which started about 5:15 p.m., likely was sparked by a cigarette discarded on a patio, fire and police officials said.

The next day, authorities deemed the building uninhabitable because crews found asbestos in the drywall mud. Testing and preliminary clean-up work has been ongoing since.

"The scene has been turned over to the property management company and insurance company," Lowrey said.

The Hill House apartment fire is the most recent of at least four major apartment fires in Boulder since January 2006.

On Jan. 22, 2006, a building at 2990 Shadow Creek Drive in the Gold Run condominium complex caught fire about 7 a.m., displacing about 75 residents from 36 units.

This January, lawsuits started surfacing seeking damages from the 2-year-old blaze. At least three insurance companies have gone to court seeking more than $500,000 from those suspected of starting the fire.

On Oct. 26, 2007, two separate buildings in the Gold Run complex -- at 2800 and 2802 Sundown Lane -- caught fire, displacing about 60 people from 36 units.

Tenants in 18 of the affected units have just been given the go-ahead to move back in.

On May 11, the Holly apartment complex at 1821 22nd St. was burned on one side after a fire started in a bush outside the building. Dozens of tenants in all of the building's 31 units were displaced after asbestos was found in the air, and they've still not been allowed back.

Officials with Matrix Real Estate, the property's manager, didn't return phone calls from the Camera on Monday.