patterns of writing on the face of the envelopes or the mailings. there are indicators of suspicious behavior that at least begins the suspicious inquiry as to what is going on here. >> reporter: nader says whether it is discovered on sending or receiving end, any suspicious package is quickly and carefully isolated. a key question tonight, how would some of the mail bombs this week have gone through the mail system to their destinations without cancellation marks on the stamps. nader says the cancellation marks are done by machines that are not fool proof. >> it may not show up at all if it goes through a machine and it is not the right dimensions. >> reporter: postal officials say it is not just the postal inspection police on the lookout. some 600,000 postal employees across the u.s. are now in effect investigators. are they in danger from possibly more bombs in the system? >> if it doesn't get identified and makes it through the system and somebody unintentionally mishandles it, the devices