The US authorities claim he deleted critical files from operating
The following is from McKinnon's Wiki page. I have to admit you were right, it is nothing like a letter bomb. No letter bomb could do $700,000 in damage.



systems, which shut down the US Army's Military District of Washington network of 2,000 computers for 24 hours, as well as deleting US Navy Weapons logs, rendering a naval base's network of 300 computers inoperable after the September 11th terrorist attacks. McKinnon is also accused of copying data, account files and passwords onto his own computer. US authorities claim the cost of tracking and correcting the problems he caused was over $700,000.[3]

While not admitting that it constituted evidence of destruction, McKinnon did admit leaving a threat on one computer:

    US foreign policy is akin to Government-sponsored terrorism these days … It was not a mistake that there was a huge security stand down on September 11 last year … I am SOLO. I will continue to disrupt at the highest levels … [4]