It's a little daunting to receive a large envelope from the FBI, until you take a closer look and realize that things are not what they seem, and this FBI is from Olivia Dunham's world in FRINGE (seen here with Phillip Broyles).
The first clue is the Frog glyph stamp, which is obviously not real and is postmarked FRINGE ON FOX 09 SEP 2008 TUE 9/10 C - the FRINGE premiere date. Another interesting detail is the braille at the bottom of the envelope, which translates to the Fibonacci numbers: 8 13 21 34 55
As previously reported, the envelope contains a letter, a digital voice recorder, "evidence" of The Pattern, and a DVD of the pilot episode. Here are some better photos of the contents: