A fast-moving fire killed four children and critically injured their mother early Tuesday, hours after firefighters investigated a smell of smoke. A neighbor helped two other children escape.

The Bergen County (search) prosecutor's arson investigation unit was at the home in the New York City suburb but authorities said they had not determined whether the fire was set.

The two children who escaped, girls ages 14 and 7, were rescued by a neighbor who propped a ladder against the burning house. The children were being checked at a hospital.

Fire Chief John Bauer said his department got a call Monday night reporting smoke in the basement of the three-story brick house, but firefighters investigated for a half-hour and couldn't find anything wrong. At least three smoke detectors inside the house were functioning properly, he said.

The family's nanny, sleeping on a couch on the first floor, was awakened by heavy smoke early Tuesday, police Lt. Norman Levine said.

"She woke up and yelled up to the mother. Apparently they couldn't make it down the stairs. They tried to get out the windows, but only a few of them made it," Levine said.

The mother was trapped in a second-floor bathroom and was rescued by police and firefighters who smashed out a small window. Phyllis Seidenfeld was listed in critical condition at St. Barnabas Medical Center (search) in Livingston, hospital spokeswoman Robin Lally said.

The dead children, whose names were not released, were boys ages 15, 6 and 4, and a 5-year-old girl, Levine said.

The children's father was not at home, he said. Broadcast reports said the couple was divorced.