In the month of May 2001 a very long and detailed threat was posted on &TOTSE.com a BBS which was a former dialup BBS Forum for conspiracy and Ufology and other subjects.
This thread of a threat detailed how planes would be highjacked and flown into the WTC as well as in Washington such as the White House and Pentagon.
There was also an incident following up the same week as 9-11-01 in the same country in the city of Antioch California (TOTSE was ran out of Walnut Creek) of an Anthrax scare in which Police Officers reported small planes flying during a no fly period.
related link on a forum discussing the events