I believe the FBI did need to get involved in the investigation surrounding Chris Christie's Bridgegate scandal. Apparently this was a pretty bad decision on Christie's part and apparently he knew there would be consequences. The FBI should be involved so the reasoning behind it can be thoroughly understood by the government.
When a local politician, which is what Christie is, no more no less, gets involved in a scandal so ridiculously bad as Bridgegate, the federal government needs to step in to see if there has been any further abuse of power. Christie's wielding of his governmental power has gone out of control.
I don't think any criminal activity occurred that would have required the FBI to investigate, I think what happened was a matter of politics or a matter of mis-communication and while it was unfortunate I don't think there needs to be criminal investigations, I think people who were effected by it should be able to sue for civil damages if they can prove that the closing of the bridge meaningfully harmed their business.
No, the FBI does not need to get involved in the investigation surrounding Chris Christie's Bridgegate scandal, because we all know that Christie was involved. Those who are going to deny it are going to deny it no matter how much evidence there is. The New Jersey voters need to find a new governor.