"The modern world of technology, because there is more information out there, it's that much more important for our people to be vigilant and for us to be vigilant," said Harlow of the U.S. Marshals Service.

Vigilance plays a role, but Shur suggested another factor may also play an important role.

"Luck, you have to throw in some luck," he said. "You can't go this many years with this many witnesses without some luck in this thing."