Yes, they do, but 1997 was not the beginning of the plot. It dates back to at least 1995 when Phillipenes police uncovered Ramzi Yousef's bomb-making lab. The officers found plans to fly planes into buildings. In fact, one of the lead officers told US authorities of the plan, but it never went anywhere within the FBI. Also, in 1996, Khalid Sheikh Mohammad discussed the plot at length with UBL.
"The officers found plans to fly planes into buildings. In fact, one of the lead officers told US authorities of the plan"(nwctwx)
Col. Mendoza