Someone (or someones) had to configure a message for each victim, they had to write the script to send all the tweets simultaneously, they probably had to test the script, they had to execute it. To me, that says they had enough time to think about what they were doing and weren't racing a very short expiration clock.

If I were at twitter I might try to investigate by looking for accounts that they might have used to test their script. If you could look for something like multiple accounts tweeting the same thing within 1 minute, in the past couple weeks, you could turn up some candidates for test accounts. You could further refine that by checking the messages sent, follower counts, etc. Maybe the hacker will leave behind clues on the script test account.