Honestly, it would run counter to the FBI's interest to not divulge flaws that invalidate the anonymity guarantees that tor provides. Tor was originally developed to mask the identity of USian spooks performing intelligence operations and will continue to be used in that role for the foreseeable future. [0]

Like any FBI spokesman would say, things that make it harder for law enforcement to engage in surreptitious observation of targets endanger us all.[1]

[0] https://news.ycombinator.com/item?id=8610532

[1] NB: I neither agree with nor endorse this claim that is all-too-often made.