A 32-year-old Alexandria woman whose body was found Monday died from a "lethal level of carbon monoxide in the bloodstream," apparently when gas from a basement boiler seeped into her apartment, the Alexandria medical examiner said yesterday.

City officials are still investigating the death of Dierdre Gallery, whose body was found by a security guard who entered her third-floor apartment at 4921 Seminary Rd. after friends expressed concern.

Shortly after midnight Sunday, the maintenance staff of Southern Towers apartments was called with a complaint about a gas odor, police said. A worker discovered fumes in the boiler room and a broken seal on the boiler, according to Lucy Crockett, spokeswoman for the Alexandria Public Safety Department. The seal was then replaced, she said.

"We believe there was a collection of fumes in the boiler room on the basement level and an escape of the fumes through part of the building to the victim's apartment," she said.

But the investigation is a "long ways from being completed," said William H. Pennell, administrator of code enforcement. "It's one of those situations where, if you get all the right conditions, something bad happens."

Pennell, representatives of the health, police and fire departments and Washington Gas Light Company ran safety checks Tuesday and found the building to be safe, Pennell said. A state inspector and a mechanical engineer will check the building soon, he said.

"The buildings were built at a time when the codes were different," said Pennell.

There are 2,400 units in the five buildings that make up Southern Towers. The complex is managed and partly owned by Mark Winkler Management in Alexandria.