Drive-by-wire steering is a very new thing though; cars with that are just coming out this summer. So unless the supposed assassins used a very sophisticated device, they would have had to of settled for hacking the electronic throttle control, and merely speed the car up and hoped he would crash without steering it for him. Of course doing that much wouldn't be very difficult. You wouldn't hack into the car via some wireless signal, or even control it wirelessly. You'd just replace the ECU when he wasn't around, and program it to trigger at a time and place (GPS), or when it received a wireless on/off trigger signal that would be pressed. I could imagine the CIA having that in their arsenal of plausibly deniable assassination tools.
It would be dumb to have the FBI make any overt moves right before doing it though, as it obviously raises suspicion. Of course assassinating a journalist is such a serious offense, that I would expect the number of people knowing about this supposed plot to be rather small, and therefore they might not be able to control the FBI. If it ever got out, it would be riots in the streets time.
There is plenty of motive though. If Hastings doesn't top the list of journalists that the military dislikes, then he's pretty damn close to it. He wrote the article that got McChrystal fired which obviously burned some bridges. His last article was very critical over the domestic spying, and no doubt his next story involved that. He had a relationship with Wikileaks-- which is an organization classified as an 'enemy' by the US government. I could imagine some sort of twisted logic they might use involving justification in order to protect the programs that protect us from terrorism, and that the ends would justify the means in their minds; similar logic was used to torture and invade a country that had done nothing to us.
I seriously doubt foul play was involved though.