The causes of two recent deadly Minneapolis fires remain mysteries, reflecting the difficulty investigators have in pinpointing the origins of destructive blazes in an era of exacting standards.
City fire investigators are still trying to determine what led to the New Year's Day explosion that killed three people in a Cedar-Riverside apartment building. They also were unable to determine what sparked the Feb. 14 blaze that killed five children in a north Minneapolis duplex this month.
About a third of the city's fire investigations end in uncertainty as tracing the cause of a fire is harder than it might seem, Minneapolis Police Sgt. Sean McKenna said.
"A fire investigation is often similar to an archaeological dig," McKenna said.
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In the case of the north Minneapolis fire, which killed five children under age 10, investigators had to sift through catastrophic damage.
"This a molten mass of some unknown metal and some pieces of flooring," McKenna said as he pointed to a photo on his computer screen.
The mass, which investigators found amid the ashes of the duplex, appears to be what's left of the space heater the children's father said was running when the fire began. It was sitting in the living room/dining room area on the second floor of the duplex. The investigation concluded that's where the fire started.
But McKenna said it's impossible to say for certain whether the space heater sparked the blaze.
"That's the $10,000 question. I don't know," he said. "And that's why this can be very difficult, and why fires are often left in the undetermined category even though we don't believe any foul play occurred."
McKenna said there are at least four ways the fire could have started: The heater could have been defective or damaged in a way that caused it to malfunction. It also could have been placed too close to a piece of furniture. Another possibility is that the building's wiring was faulty.
Narrowing the possible triggers to a single, definitive cause is difficult, because fires destroy evidence. The heater is now a misshapen blob of metal. The room is charred top to bottom, and there's a six-foot wide hole in the ceiling.
In general, the more severe the fire, the harder it can be to pinpoint its cause, said former Minneapolis Fire Chief Bonnie Bleskachek, one of three fire investigators in the department.
"Sometimes a building will actually collapse, and you've got a basement full of water, and that's what you have to look at," she said. "Trying to determine an area of origin that might have been on the third floor becomes very difficult."
That's one of the challenges investigators are likely facing as they sift through the wreckage of January's explosion at a three-story Cedar-Riverside apartment building. Thanks to subzero temperatures, the water used to extinguish it is a blanket of ice.
Last year, Minneapolis investigated almost 400 fires. About a third of those investigations were inconclusive. The state Fire Marshal's Office reports similar numbers for its investigations.
Jamie Novak, an investigator with the St. Paul Fire Department, said he has seen an increase in the percentage of fire causes deemed undetermined during his nearly 30-year career. That's because fire investigation, once more of an art, has become increasingly scientific, he said.
"When I first started, it was stuff passed on from investigator to investigator," Novak said. "A lot of stuff didn't really have a lot of scientific testing behind it. I think fire investigating has come out of the Dark Ages over the last 15-20 years."
A major turning point came in 1992, when the National Fire Protection Association put out the first country-wide standards for arson investigations. Novak said the standards called for a systematic approach based on scientific research, which has raised the bar.
"You don't just call a fire electrical, because you can't think of nothing else, [and say] 'Let's go home. It's cold out,'" he said. "So it's made us have a higher standard before you can call a fire."
The standards continue to rise. Until 2011, they still allowed investigators to label a fire as "arson," simply by ruling out all conceivable accidental causes. Today, investigators need more evidence.
If that means more uncertainty, Sgt. Sean McKenna is fine with that.
"I would not want to have the cannon of the justice system pointed at me if I didn't do anything wrong," he said. "And I think it's just more fair to the citizens."
Even though the cause of the Colfax Avenue fire has been labeled "undetermined," the case isn't officially closed, yet. Private investigators, working on behalf of insurance companies, are conducting hi-tech tests on the remains of the space heater, aiming to determine precisely what caused the fire.