Trump was told he wasn't a target of the investigation. Being the target of an investigation is a specific legal term. If the investigation progressed (subpoenas were issues for various Flynn associates yesterday) to the point where guys like Flynn were indicted and then offered a deal to tell the truth then Trump would become a target of the investigation. But these investigations take years and Trump holds the highest office in the land. It's not something that can happen overnight. I'm pretty sure nobody will roll over on him as far as Russia goes and the worst that will happen is that he just looks incompetent for letting Russia run his campaign to the extent they did. What he really needs to worry about are the money laundering investigations and his ties to various crooks. Comey getting fired will supercharge the Russia story in the media and stuff that has slipped under the radar like Trump casinos being charged/paying fines for laundering Russian money will get an airing. Lots of leaks to come the next few weeks anyway.