There was an outright denial that the Tampa teen stole and crashed a plane as any sort of terrorist act yet that is exactly what his suicide note says (with the word terrorist crossed out by someone) and praise for Bin Laden.
The letter was not published for several months.
I won't ever jump to a conclusion that something is the work of terrorists or saboteurs but I also refuse to believe any quick "not a terrorist act" declaration. CLEARLY there has been at least one incident where the authorities DID know that is what it was.
Also consider the messages from the DC snipers yet Moose had the public looking for white guys in a white van. They knew there were terrorists involved.
You can also add the terrorist incident at Los Angeles International Airport to your list. (7/4/2002) The City was adamant within hours that it was not terrorist related, when in fact the gunman was a member of the Muslim Brotherhood in Egypt, and sent his family back days prior to the incident. The FBI later confirmed it was classified as a terrorist incident.
A RAND study was ignored by the City and Department of Airports, stating the El Al counter at LAX was the number one target for terrorism in Los Angeles, and additional well-armed officers should be placed there.