MD82 crash scene photo
 

Never ceases to amaze me how obliterated an aircraft becomes in this type of accident. God bless those aboard and their families.

I don't understand how a crash investigator can piece something like that together.

Witness marks tell much of the story about the physics of the impact. The CVR can provide details about the aircraft condition prior to imapct. Then eye witness reports, ATC transcripts, WX reports, AC maint. history, and many other factors will provide a big picture. In the end it comes down to the experience of the investigation team. Lots of talent out there looking over "high yuck factor" wrecks.