From the History Commons Groups blog:
Many entries have been added to the Complete 9/11 Timeline at History Commons, which describe a variety of events relating to the 9/11 attacks, with numerous entries revealing new details of the events of September 11, 2001.
Training Exercises Were Held in the Run-Up to 9/11
Several new timeline entries describe training exercises that were held in the years and months before 9/11. In May 1998, an exercise was held at the Pentagon called Cloudy Office, which was based around the scenario of a terrorist attack at the Pentagon involving chemical weapons.
In February 2001, the FBI’s Washington, DC, field office started sponsoring training with local fire department and law enforcement commanders on how emergency response workers and the FBI should coordinate their actions if there was a terrorist attack in the Washington area. Just two or three days before 9/11, the Washington field office and various other agencies held an exercise in Northern Virginia, based around the scenario of a terrorist attack involving chemical weapons.
And United Airlines, which had two of its planes hijacked on September 11, held a surprise exercise 12 days before 9/11 in which employees were led to believe one of their planes had crashed.