Computers from colombian guerrilla and interpol

Hi.. you might know the colombian goverment killed a leader of the colombian's guerrilla "FARC" (bombs and everything) and took the portable computer from that leader (Raul Reyes). Almost two weeks later the goverment send the whole computer to the Interpol and, after a while, the Interpol certified the data in the hard disk was read but was not modified after the attack.
Is it possible nowadays to certify something like that? Is it possible to modify a hard disk data and leave no clue of it?