ROBERT SIEGEL, HOST:

It's now five days into the search for survivors of the massive landslide in Oso in Washington's Snohomish County. National Guard Troops are combing the area with emergency extraction teams. The unofficial death toll so far is now 24, and authorities are promising more clarity tomorrow on the list of missing people. Some 176 persons are unaccounted for but the real number is thought to be lower than that.

This area was not new to landslides. They've happened repeatedly on this hill over the decades. And in fact, the slope was once referred to as Slide Hill. Snohomish County Emergency Management Director John Pennington said on Monday the area had been considered very safe. Today he said this.

JOHN PENNINGTON: People knew that this is a landslide-prone area. Sometimes big events just happen, sometimes large events that nobody sees happens.

SIEGEL: So, should the people who live in Oso have known this might happen? And if so, who should have told them?

Well, we're going to ask Ken Armstrong. He's an investigative reporter for The Seattle Times and he's written about the history of this slope. Welcome to the program.

KEN ARMSTRONG: Thank you, Robert. Thanks for having me on.

SIEGEL: And first, give us some more detail on how many times this particular hill has given way and did it destroy homes before?

ARMSTRONG: It has given away so many times that it is referred to as Slide Hill, as you mentioned before. It gave away in 1949, in '51, in '67, in '88, in 2006. There have been reports written over the years from geologists, hydrologists who have said that this hill is constantly moving. And that whatever measures are taken to minimize the risks posed by it, they're likely to fail in the long run.

SIEGEL: Which raises the question, pretty important and obvious one: Why were people building homes on this hill?

ARMSTRONG: It's the same question I suppose that can be posed to why do people build on the Outer Banks of North Carolina, or in areas of Southern California hit by wildfires. I think there is a certain assumption of risk. One of the most jarring images here was provided by a geologist. In 2006, there was a large slide. And even before the debris had been removed, he went out and visited and saw carpenters building new homes.

It's up to the county to decide whether to issue building permits. And here, the county has continued to issue them even after these slides.

SIEGEL: As you say, the county was issuing building permits. People still presumably would need insurance. People who were real estate agents involved in selling properties would have to come clean about the history of the property. I mean other people might have been accountable, don't you think?

ARMSTRONG: Presumably. My guess is that every homeowner would have a different story to tell. But we've seen minutes of some meetings where this very discussion took place, and people were concerned that there was a lack of disclosure.

SIEGEL: Now, we heard John Pennington at the beginning of this conversation, say that this was an area that had known landslides. On Monday he said, I'm quoting now, "It was considered very safe. This was a completely unforeseen slide. This came out of nowhere." The only way I could imagine reconciling those two comments would be the magnitude of the slide. Can one plead that or are these statements just totally irreconcilable?

ARMSTRONG: I think you can plead that. Certainly another slide, even another very large slide, was not unforeseen. But when you look at the scarp on the side, which would be the face of the cliff that was created by the slide, this one is 600 feet tall. The largest slide previously had a scarp of 150 feet. This one was certainly bigger than anything that had been seen. But if you look at the reports that have been documented over the years, scientists have been warning of something like this.

SIEGEL: Ken Armstrong, thanks for talking with us today.

ARMSTRONG: Thank you, Robert.

SIEGEL: That's Ken Armstrong, an investigative reporter for The Seattle Times.

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