1993 WTC bombing
On 1993-02-26, a car bomb was detonated in the parking deck below tower 1 of the World Trade Center (WTC1) in New York City. The bomb was apparently intended to topple WTC1 and send it crashing into WTC2, destroying them both; WTC1 remained standing and was repaired and returned to service not long afterward.
We haven't yet found the evidence or explanation of how they determined that the bomb was intended to topple the tower, although this claim in the Wikipedia article does have citations (one is a book; need to read the internet link more carefully, and perhaps obtain the book).
It is often mentioned that the FBI was aware of the plot well in advance and actively failed to prevent the bombing, but so far we only have one source for this (the 1993 NYT article, based on testimony from an informer/infiltrator). Are there any other sources?