It seems that this didn't look like @flyingwithfish's words and that the intent of the message seemed out of character for him. I'm not particularly concerned with whether the federal agent took his computer/phone while he was logged in to Twitter and posted something to Twitter in his name, or handed the computer back to him and coerced him into pressing 'enter'.

Until people get used to the idea of cryptographically signing their published words, never giving up their signing key, (and we'll need software to make that easy, and available not just with e-mail but with services like Twitter), we'll always run the risk of something being "said" and attributed to the wrong person. What seems to have happened here is similar to the investigator having picked up some of Fish's letterhead and mailed a letter with it in an attempt to discover his source, or wrote the same letter then coerced Fish into dropping it in the mailbox. Who cares which it was?