Nafeez Ahmed has created lengthy lists of people who apparently warned of the 9/11 attacks. One of these is apparently a Professor Garth L Nicolson, and his wife, Nancy:
Was this covered up? Ignored by the media, the FBI, the 9/11 Commission? As it happens: no. The FBI did interview the Nicolsons, the Commission checked the documents, and the results are now available for all to read.
If this is accurate then the Nicolsons don't appear to be the most credible of witnesses. Considerably more information and verification is required before we would feel able to take them seriously.