The now-unsealed records show that on March 26, a correction was made to the indictment of one of the men, Thomas Piatek, because his middle name was listed wrong in the indictment and arrest warrant.
In the document noting the correction, this passage appears:
"The evidence presented before the Grand Jury ... included the sworn testimony of a law enforcement officer that the person pictured therein is the person known by a Cooperating Witness and an undercover FBI agent ... "
So there you have it -- there was an undercover agent and a cooperating witness who positively identified Piatek. We're pretty sure there's a good story behind this one ...