Carbon monoxide leak in

An official from the city's Inspectional Services Department said yesterday that the source of the carbon monoxide leak that forced the evacuation of an Allston apartment building last week was an underground fire, not a faulty heater, as city fire officials maintain.

But a spokesman for the Fire Department, Scott Salman, said city firefighters did not put out a fire at the Brighton Avenue complex early Thursday and did not smell any smoke that would have led them to believe the source was a fire.

"As far as the Fire Department is concerned, the first determination is correct that it was a faulty ventilation of a heater," Salman said. An underground fire involving burning wires is not likely to cause a leak with the intensity of the one last week, when detectors indicated the carbon monoxide levels were dangerously high, he said.

Salman acknowledged that officials from Inspectional Services remained on the scene after the firefighters had left.

Lisa Timberlake, a spokeswoman for the city's Inspectional Services Department, said the agency's investigators "determined it was some sort of underground fire that caused the fumes to back up."

NSTAR spokesman Mike Durand said the utility's employees were working in the area that night on a nearby street, but there was no indication their work contributed to the leak.

A representative of Brighton Realty, the company that manages the building, did not return a call seeking comment.

One of the four occupants of a basement apartment called 911 after they were awakened by a blaring carbon monoxide detector. Firefighters determined carbon monoxide levels were high enough to justify evacuating the building.

The tenants of the apartment where the carbon monoxide levels were highest were treated and released at Caritas St. Elizabeth's Medical Center. Inspectional Services Department officials also later determined that the four basement units were illegal because Brighton Realty had not obtained the proper permits.

The occupants of the apartment where the carbon monoxide levels were the highest were moved to a hotel temporarily. They do not plan to return, said the father of one of the residents.

In a hearing yesterday, the inspectional services department said all tenants could continue to live there, but they ordered another round of inspections in the basement units today, said Dion Irish, assistant commissioner for the department.

Even if violations are found during that inspection, the tenants could be allowed to stay as long as the apartment is not deemed unsafe.

Globe correspondent Emma Stickgold contributed to this report. Scott Goldstein can be reached at sgoldstein@globe.com.