The FBI takes over the case because they said these murders constitute domestic terrorism, however the facts of the case don't support that conclusion. The purpose of domestic terrorism is to incite public fear and intimidate or coerce the civilian population and/or the government of the United States, and these acts are carried out by U.S. citizens. At that point in the investigation there were no suspects, so no way to know if the person(s) responsible was from the U.S. or another country. But the main component of terrorism is missing: public knowledge of the crimes, the public can't be terrorized if they don't know about the cases of poisonings. If this was a case of domestic terrorism the person or group of people responsible would have made some kind of public statement claiming responsibility, this is usually done by sending a statement and/or manifesto to the media. All the evidence so far points to a serial killer, including the fact that the timing of the poisonings coincided with flu season and a poison that mimics the symptoms of the flu was chosen so that it wouldn't have been easily noticed. Some killers get off on killing people without anyone knowing, it's a form of god complex.