I agree GA_Peach, I am confident as well. I have seen episodes on tv where the Dallas PD drove day long routes when a suspect claimed they were out of town, just to validate drive time "possibility" (I wonder if they just use google map estimates these days). Honestly, there are so many ways around it, even a corroborated alibi very rarely will rule someone out. Outside of something like xyz was at a work, where they badged in, they are on video and have 20 other independent people saying we were all in a room in a meeting at the time of the murder, I think everyone is likely still in the pool. My guess is they put in the effort to dig deep when they have other reasons to believe you are involved. Most LE don't have the resources to put in the man hours towards testing every piece of every POIs alibi. They do basic checks, and if nothing seems off, put you in the category of corroborated and move forward knowing full well everyone in that group could still be involved. If something doesn't check, the real digging starts. They also run in to issues with the time of the murder in this case. I imagine lots of people they questioned just said 'home in bed'. Not much way to validate that, but most likely those people are being honest.
And like others mentioned, even if your alibi holds up to all of the investigating and they KNOW you weren't there, LE knows you could still be involved, you just didn't pull the trigger, so to speak.
This alibi discussion brings up another thought for me. I know several people have an eye on CT. Has anyone heard her alibi? My first thought is she has little kid(s) and a husband that we already know was up getting ready for his own classes. Who would have stayed with her kid(s)? And that would mean you would have to tell AT what you were doing, so now 2 people are basically willing to commit murder.
One last thought, someone mentioned BB sending a look-a-like to Biloxi. This seems unlikely to me for a few reasons, but mostly because the more people you tell\involve, the more likely you are to get caught. People talk, especially those involved in that manner. If LE is able to figure out who, they almost always offer that person MUCH reduced charges in turn for them spilling the beans. As a matter of fact, even if you were a hired hitman and did pull the trigger, you almost always get a lighter sentence than the person who hired you.