1.  They've been tracking mail for years.  This is just the next step.  

2.  I don't doubt for a minute they've been opening mail for a while, because this isn't the sort of thing Bushie and his counsel would dream up for a signing statement that responds to a statute explicitly prohibiting opening mail without a warrant.  The prohibitory statute would not have come about absent an existing problem - societies (of whatever stripe) don't write into their legal codes prohibitions on problems which don't exist.

Back after the still-unsolved anthrax attacks on 2001-2002, the FBI sent agents out, tracking down the mail which was near-in-the-mail-stream to the affected letters.  They were looking to find whether there had been any cross-contamination of other mail, and also to recover any infected/affected mail.  A friend worked in a multi-story office building which had the devoted attention of at least one cheerful, courteous young FBI agent during this needle-in-haystack search.  This agent went from one office to the next office for over a week, inquiring in each whether they had received any mail during a specified period from addresses in South Jersey.  Remember, the mail had come from public boxes in South Jersey* and had been processed through a bulk center in that area, which had to be shut down from contamination for an extended period.  It took time for the search, because many of these offices (like so many offices) had large quantities of paper and files to go through, one-by-one.  

At one point, the agent came to a law office, where arose the issue of "we're a law office and surely have confidential client communications in our files.  We cannot ethically allow the FBI to root through them, no matter how noble the purpose."  Agent, I'm told, was polite and reached a compromise where agent said what they were looking for and law office agreed to look and advise if anything responsive was there (it turned out there wasn't anything).

In the course of the discussion, I'm told, agent was asked why, out of all the office buildings in the world, the FBI had chosen this one for its attentions.  Agent's answer was vague but along the lines that the machinery for sorting mail had the capability to know where the mail went, and they knew mail addressed to this particular office building was near the poisoned letter, in the mail stream.

I'm speculating, now, but I suppose it has something to do with the bar codes in postmarks, etc.  I mean, you don't really think every letter gets sent on its way by human hands and eyes?

Congress needs to pull together veto-proof majorities to pass a law rendering signing statements unlawful, of no legal import whatever, and stating clearly that no court can consider or in any way give any deference to a signing statement for any purpose whatsoever. Make them a legal nullity.

Didn't CNN get the message from the last election....that the American public has rejected the right-wing extremist agenda?
Why is CNN giving prime-time every night to the ultra-right-wing extremist, Glenn Beck?

Please take just 10 seconds to click on this link and send a message to  CNN telling them we don't want the airwaves innundated by this idiotic
low-grade right-wing propaganda!