Their new house in North Branch, Minn., was getting chilly when Mitch Carlson and his fiancée, Penny Pliscott, returned before dawn Tuesday. They didn't know why their propane boiler was malfunctioning, only that it came at an inconvenient time. The family was sick.

About 6:30 a.m. the couple went downstairs to their 17-year-old son's bedroom.

There, sprawled on the floor, was Andrew Carlson. It was obvious that he was dead, Mitch Carlson said Thursday.

Andrew Carlson's death capped a chain of events that began with the family falling sick over the weekend, while carbon monoxide gas rose to lethal levels in their new home on 412th Street.

The poisonous gas made them sluggish and disoriented. Then it led police to jail a staggering, slurring Mitch Carlson for allegedly driving while intoxicated just hours before he found his son dead.

Now, the family and investigators question how a new propane boiler used to heat the house may have gone awry. The family had moved in after an inspector approved the house for occupancy last Friday.

On Thursday, nobody had yet determined exactly what time Andy died after he went to bed in his basement bedroom near the boiler, according to the Minnesota regional medical examiner's office in Hastings.

What is clear is that carbon monoxide killed him.

In North Branch, a fast-growing city in Chisago County, Mitch Carlson and his family were eager to move into their new home. He had bought the components for the NY Thermal Trinity boiler, made in Canada, and had a contractor hook it up. It heats water for radiant heating.

On Friday, city inspector Mark Jones looked over the boiler and other mechanicals in the house, and signed a permit of occupancy, according to city records. City officials declined to comment Thursday.

Mitch Carlson, 47; his fiancée Penny Pliscott, 43; their son Andrew, and her sons Phillip Bartholomew, 10, and James Bartholomew, 12, moved in that weekend.

Mitch Carlson and Pliscott had gotten back together after years apart and were planning to be married today. But headaches and vomiting set in, and the entire family thought they had the flu.

Not long after midnight Tuesday, Mitch Carlson drove from home about 3 miles to a Holiday gas station to buy cigarettes and had what he described as a minor fender-bender near the pumps.

"I hit the dispenser that you put the washing fluid and squeegee in," Carlson said.

But it wasn't until he went to pay that he realized that something was seriously wrong.

"I went to write out a check, and my brain wasn't working. I really didn't realize what was going on," Carlson said.

Police Sgt. Rick Sapp arrived and witnessed Carlson stumbling, slurring words and having a hard time following directions. Yet a breath test showed only a tiny amount of alcohol.

At Wyoming Fairview Hospital, workers drew blood for alcohol testing. Sapp took Carlson to jail, and he was released about 3:30 a.m. Nobody suspected that carbon monoxide was the problem.