You would think that in this day and age they could have caught this guy without and witnesses. The IP tracks to a computer. Video cameras should be recording who is using the computers.
He should not have been able to use that computer, should have required a CAC card...if he had one it should have tracked his use.
You are also correct that there should have been video cameras.
I am scratching my head at the weak security...I retired 4 years ago from a government facility and the security there was far more intense than what this sounds like. This sounds like he wandered in and used the computer with no issue. It is harder than that to use a computer at a public library.
There are a variety of networking methodologies that would easily obscure the identity of the computer or other device that was used.