Homeland Security keeping America safe from... dead Nazi war criminals?

Here's an interesting story from Reuters about the search for the diaries of Nazi war criminal Alfred Rosenberg.

Rosenberg was one of the most odious Nazi ideologues, who got considerably less than he deserved when he was sentenced to death by hanging at the Nuremberg trials.

What caught my eye was the first sentence in the penultimate paragraph; "...an agent from Homeland Security Investigations tried to find the missing diary pages..."

I thought it was the mandate of Homeland Security to keep America safe from terror?

Talk about mission creep!