Writing a novel about an event after it happens is possible. It might even be substantially accurate. Doing the same before the event is straining credibility. Ed Steele mentions the fact at OKC Massacre which he wrote before the 9/11 Job. It checks out. See Amazon's review at The Final Jihad.
The massacre really did happen. After that things get strange.
The only really satisfactory explanation is that it was set up by the American
government.
The bomb or bombs did make a mess of the place. A blast from an ANFO bomb
sweeping up the face might have done the job. Cutting charges properly placed
would have dropped the whole building. Whether the Tim's hypothetical bomb was
anything like big enough is another matter.