"I will blow up an abortion clinic in the process."
Go ahead. Prove you're not merely an Internet Tough Guy, Sambo83.
"The FBI do have a tip line, don't they?"
"can we phone the FBI on this guy?"
The CIA & NSA (with help from MI6 & their GCHQ here in the UK) will have what this Internet Tough Guy said flagged up (after all, GCHQ's supercomputers, and their ECHELON, scans all telecommunications), and his IP address, just in case. After all, remember Jake Brahm?:
The FBI, as well as the CIA & NSA will have tightened things up, internet-wise, since then.
If, when the time comes, you find yourself face down on the floor, hands zip-wired behind your back, and looking down the business ends of several H&K MP5s, Sambo83, I doubt the FBI agents wielding such will accept 'It was just a joke, honest!'
Oklahoma City in 1995, and 9/11 has ensured they'll never have a sense of humour.