This is so awful. My best friend used to work for the radio station in Humboldt, and his morning show co host/play by play guy for the Broncos is among the people that died. I used to play hockey with another kid's brother. Just an awful awful thing to happen.
As for the intersection, it sounds like the semi completely blew the intersection. Dunno if he was asleep or didn't notice the stop sign or what. I'd also speculate he was coming from the east, where there's a yard with trees on that corner of the intersection. I'm guessing those trees prevented the drivers from seeing each other, in addition to the semi not stopping. The Google Map street view of that intersection is pretty chilling, you can see 5 crosses in the ditch for people who have died in that intersection previously. Just a terrible stupid accident.