DHS? CIA? I, too, have been visited frequently by PSI.  Snips from a couple today posted below.  (Also had an unusually high cluster of intertwined PSI / Google / ARPA visits (not google searches, google mountain view) recently which seemed related.

How to block Spam Bot (Spider) from PSI (Performance Systems International) | my-whiteboard.

I have a high school friend who works in the Department of Homeland Security and he tells me that the bot in question is part of the Homeland Securitys team. Whether it is a person or crawler I dont know but it has gone to your domian on a keyword basis. Be afraid? Only if you have something to hide.