OK. It would be easy enough to destroy a hard drive with a hammer and screwdriver. If the discs are broken, it is very doubtful that anything important would be recovered. Since they reference Yahoo, I assume that his ISP used them or he had used Yahoo as the primary search engine. It is not credible to say that his browsing habbits are no longer available, they certainly are. All ISP's are required to record that data and much more.
"authorities" are not the best place to attempt to recover data. Good data recovery companies are better at it than the GOV, with the exception of the NSA.
we had a hard drive die on a dell pc. since the computer was at the time fairly new we contacted dell for a new one. now this hard drive had financial info etc. on it and we refused to give it back to Dell (they gave us a hard time about it but finally relented)
anyway we did a ton of research on how to destroy a hard drive and it is not easy... my husband works in a hospital and honestly the best way is to bring it with you into the MRI room as the (very powerful) magnet will erase all the data
either that or a hydraulic press.
regarding ISP addresses and such my understanding is there are some pretty sophisticated ways to hide where you are going...especially amongst the computer savvy kids ( like Adam apparently was) who may decide to go to some very bad places like the deep web ( incidentally a whole FBI unit is dedicated to monitoring the deep web.)
I wonder if they are looking there for prior activity to look for others who may be similarly inclined
In this case, Yahoo is just a news source.