The tweets have just been pulled, but this morning the Fox News Politics twitter account was reporting that the president had been shot twice in Iowa and killed. Obama is, of course, totally fine. The account was hacked shortly after midnight and the messages about the president were left around three o'clock in the morning. In total there were six tweets that reported the death of the president, going so far as to wish Joe Biden luck in his assumed new role.
Originally the account was hacked by a group called the "Script Kiddies" (a derogatory term for beginner hackers) who vaguely aligned themselves with Anonymous and changed the display picture for the Fox account to their logo, according to the Huffington Post. Those tweets were deleted and replaced with the fake assassination tweets after an hour or so. The "Script Kiddies" account has also been taken down.
Gawker pointed to Think, a SUNY Stony Brook student-run magazine, that claims to have spoken with one of the members of "The Script Kiddies". They said the group consisted for former Anonymous members and they were forced to join the anti-security movement. Despite being named after beginners, they chose to target Fox, "because we figured their security would be just as much of a joke as their reporting."
Fox News released a statement today saying they are currently working with Twitter to find out who did this, and how they did it. They "regret any distress the false tweets may have created."