From the History Commons Groups blog:

A large number of new entries have been added to the Complete 9/11 Timeline at History Commons. Most of these describe events from the day of 9/11 itself, although a few entries look at pre-9/11 and post-9/11 events.

One new entry reveals that in April 2001, CIA counterterrorism chief Cofer Black warned that "something big [is] coming and that it very likely could be in the US." Then, about a month before 9/11, White House counterterrorism chief Richard Clarke visited Wall Street, to investigate the security precautions there.

Two new entries look at the time Ziad Jarrah, the alleged hijacker pilot of Flight 93 on 9/11, spent in Philadelphia. This included two days at a flight school, which refused to rent Jarrah a plane due to his inadequate piloting skills.

Some entries look at a little-known government agency called the National Communications System (NCS). The NCS happened to turn on a special backup communication system for use in emergencies for "exercise mode" the day before 9/11, and on the morning of 9/11, the CIA was briefing the NCS on the terrorist threat to the US's telecommunications infrastructure. The NCS's coordinating center subsequently played an important role in the government's response to the 9/11 attacks.

Entries reveal that special emergency transmitters carried by aircraft went off in the New York area around the time of the attacks on the World Trade Center. However, these transmitters, which are designed to go off automatically if a plane crashes, inexplicably were activated minutes before Flight 11 hit the North Tower and Flight 175 hit the South Tower.

Events at two airlines are described. At 8:29 a.m. on September 11, American Airlines learned details of the crisis from the FAA's Boston Center, and this led it to suspect that its Flight 11 was hijacked. One of its managers then instructed that air traffic controllers be told to treat the flight as an emergency. But, around the time the attacks began, American Airlines had problems communicating with the FAA's Command Center. And United Airlines, the other airline whose planes were targeted in the attacks, experienced two hours where its communications were severely impaired.

Several entries look at aircraft falsely reported to have been hijacked or in distress, including a United Airlines plane that was still at the airport; a Continental Airlines plane that signaled three times it had been hijacked; an American Airlines plane that sent a "hijack message" to the airline; a plane whose distraught pilot contacted a facility in England; and three planes that the Coast Guard reported were transmitting distress signals.

Several military exercises are known to have been underway the morning of 9/11. And when a commander at Barksdale Air Force Base, Louisiana, was first told a plane had hit the WTC, he mistakenly thought this was a simulated scenario as part of the exercise, "Global Guardian."

Other new timeline entries describe evacuations of the airports in San Francisco and Los Angeles, the original destinations of the hijacked planes.

The actions of General Henry Shelton, then the chairman of the Joint Chiefs of Staff, are detailed. Shortly before 9/11, Shelton warned other officials to "be on your toes" for a domestic terrorist attack "at any time." But on September 11, he headed off early for a NATO meeting in Europe and was flying over the Atlantic when he learned of the attacks. Shelton learned further details of the attacks in a phone call with his vice chairman, Richard Myers, but only made it back to the Pentagon late that afternoon. He was subsequently able to join Donald Rumsfeld and others for a press conference, where Rumsfeld announced that the Pentagon would be "in business tomorrow."

Another high-level official notable for being absent from his post on September 11 was Van Harp, head of the FBI's Washington, DC, field office, who was on vacation in South Carolina and only made it back to Washington late in the afternoon of 9/11.

Looking at the aftermath of 9/11, several entries describe an attempt that was made to transcribe the tape recordings of operations at NORAD's Northeast Air Defense Sector on September 11, which NORAD commander Ralph Eberhart stressed the importance of, but which led to one of the tapes being damaged. The NEADS tape recorders had already failed to record the positions of two key employees while the 9/11 attacks took place.

Source.