SEATTLE -- Some of the victims of carbon monoxide poisoning that sickened nine people in SeaTac have been upgraded to serious condition, officials at Harborview Medical Center said.

A hospital representative said three patients who were overcome Thursday evening at a SeaTac home are in the intensive care unit. Those patients had been listed in critical condition overnight.

Investigators are trying to determine why a taxi driver left his cab running after he arrived home Thursday. His wife was inside the home.

Neighbors said the couple's 26-year-old son came home and called a relative after finding his parents unconscious. They said the son then lost consciousness, too.

The relative and her two children, who live nearby, also quickly felt the effects of the carbon monoxide, neighbors said. The relative called 911.

A short time later, three sheriff's deputies arrived, and they, too, began to feel ill from the carbon monoxide.

Three of the victims underwent treatment in a hyperbaric chamber at Virginia Mason Medical Center in Seattle.

A neighbor told KIRO 7 Eyewitness News reporter Jeff Dubois that it was unusual for the cab driver to park his taxi in the garage. He usually parked in the driveway.

Police said they plan to talk to the victims to learn more about what happened.

"At this point, it doesn't seem suspicious, but we think at this point (that) it's an accident," said John Urquhart of the King County Sheriff's Department.

Neighbors said the family has lived in the home for a year or less and is relatively quiet.

The three deputies were treated and released from the hospital.