The 50 feet long shark reference?

Its a reference to the truck (a 50 foot shark).

He uses the idea of a fifty foot shark having never been seen as a symbol for a serial killer that does not follow typical serial killer profiles and therefore cannot be tracked via the standard evidentiary proceedure. "Just because you've never seen one, doesn't mean it doesn't exist" can apply do both the shark, and the hypothetical killer. The kicker is, a killer like this very well could exist and we just haven't been able to detect it because there is no pattern to detect / track...

It's also a reference to Moby Dick as Captain Ahab believes killing the whale will rid the world of evil just like O'Brian believes killing Suspect Zero will do the same.