The law firm would be putting themselves on the line for LTT if there was any further legal action, or if the subject of the investigation brought forth more evidence.
I doubt LTT is big enough to give them the incentive to do that.
Hiring a third party investigator is not the same as internal affairs. Internal affairs have only one client and little incentive to bite the hand that feeds them.
If LTT goes down after this and it comes out that the law firm missed something major or outright lied, it would call into question every investigation they've done (at least recently) and destroy their reputation.