September 11, 2001, Terrorist Attacks (Forensic Investigations Of)

Forensic investigations can occur in relative obscurity or can be front-page news. A horrific example of the latter occurred in the aftermath of the terrorist attacks on September 11, 2001.

At 8:46 A.M. on September 11, 2001, American Airlines Flight 11, hijacked from Boston's Logan Airport with 92 people on board, crashed into the upper floors of the World Trade Center's North Tower in lower Manhattan, New York. Seventeen minutes later, United Airlines Flight 175, also hijacked from Logan and with 65 people on board, crashed into the South Tower. By this time, virtually the entire nation had tuned in to witness the after-effects on television of what at first seemed a terrible accident, but was quickly revealed as a terrorist attack. Over the course of the next 85 minutes, the South Tower collapsed, followed by the collapse of the North Tower. The incident, in which nearly 3,000 people died, ranks as the worst case of mass murder in U.S. history, the worst building disaster in human history, and the largest terrorist incident in the history of the western world.

The forensic determinations that occurred in the aftermath of the tragedy were multi-pronged. Forensic inspectors sought to identify the dead and to determine the nature of the events that led to the collapse of the two buildings. Amid the carnage, investigators needed to observe the wreckage in an effort to find clues to the collapse of the buildings, while at the same time try to locate survivors.

On the same day, American Airlines Flight 77, departing from Washington, D.C.'s Dulles International Airport bound for Los Angeles, was hijacked and crashed into the Pentagon building in Washington, D.C. Another hijacked aircraft, United Airlines Flight 93, en route from Newark, New Jersey, to San Francisco, California, was also commandeered. The ultimate target of that hijacked plane remains unknown as its passengers fought back against the terrorists, crashing the plane in a field near Pittsburgh, Pennsylvania.

In the Washington and Pennsylvania crashes, the forensic investigations initially focused on identification of victims from clothing, dental evidence , fingerprint patterns, and analysis of genetic material. The goal was to ascertain the sequences of deoxyribonucleic acid (DNA ) that can be as unique as a fingerprint.

Later, forensic efforts were also geared toward determining the cause of the crashes, including searches to locate the flight data recorders that are a feature on aircraft, and analysis of the communications between the pilots and air traffic control personnel.

In New York, the forensic engineering work needed to proceed at the same time as wreckage was being cleared away. Because the priority was to uncover any survivors trapped beneath the rubble, steel beams and other rubble were removed from the site before investigators had an opportunity to inspect the structural remains on site in great detail. In retrospect, this hampered efforts to establish a cause of the towers' collapse. Ironically, one the forensic inspectors was able to assess the damage to some of the trade center's structure when a truck transporting the wreckage parked in front of his hotel.

Pieces of the structure that were recovered were instructive. For example, gouge marks on steel beams that were made by a wing and nose section of one of the planes indicated that the plane was traveling at a high rate of speed. In another example, beams were recovered with rivets still intact, which indicated that the buildings withstood the impact of the aircraft.

When the true scope of the loss of life became apparent, forensic efforts shifted to the identification of the dead. Indeed, DNA analysis was instrumental in identifying more than 1,600 victims. More than 800 of the victims were identified by their DNA alone.

As the identification of the victims came to a close, the longer term forensic engineering work continued. This work has continued until 2005, when reports on the cause of the buildings' collapse were issued.

A necessary part of the forensic engineering investigations involved the design and construction of the World Trade Center towers. Designed by architect Minoru Yamasaki—who, ironically, had a fear of heights—and engineered by Leslie Robertson and John Skilling, the 110-story towers soared 1,360 feet (415 m) above an open plaza, which made them the world's tallest buildings at the time of their completion in 1973. Whereas the Empire State Building and other older skyscrapers drew support from an interior grid of steel girders, support for the trade towers came from the exterior and the inner core. Horizontal floor trusses joined the perimeter support structure to the central area, which the engineers envisioned as a great "tube" running through the building and containing not only its support structure, but also its utilities such as elevators. This design had two advantages. First, it made the buildings extremely stable—not prone to swaying in high winds as the Empire State did. And, it left much of the interior available as rentable space.

By the 1980s, New Yorkers had become accustomed to the trade towers, which punctuated the skyline as the ultimate symbol of American and international commerce. Then, in February 1993, just months before the towers turned 20, the towers became the target of a bombing by Islamic terrorists operating a van filled with explosives . In this, the first terrorist attack, six people were killed, but the structural integrity of the towers themselves was not threatened.

When Flight 11 crashed into the North Tower of the World Trade Center at 8:46 A.M., smoke and flames began to gush from the upper stories, and workers began to evacuate the lower floors. Some, however, chose to remain at their desks. For workers on the floors above the impact area, there was no choice but to remain in place.

For 17 minutes, it was possible to assume that what had happened to the North Tower was an accident; then, Flight 175 smashed into the South Tower. Once again, smoke and flames erupted from the heights of the building, and tenants down below began a slow, but steady evacuation while others—many with no choice—stayed where they were.

Seventeen minutes after the impact, with little warning, the South Tower, succumbed to the stress caused by the fire and began to crash from the top down. As it fell, it created a vast cloud of dust and ash above and filled the streets below with noise, heat, and terror.

Shortly thereafter, the North Tower began to implode, once again crashing downward from the top. The area around what had once been the World Trade Center became smoke, ash, and dust.

In late 2001, a team of investigators that included representatives of the American Society of Civil Engineers and the Federal Emergency Management Agency (FEMA) commenced a study on the structural collapse of the towers, the details of which they made public in April 2002. In August 2002, the National Institute of Standards and Technology (NIST) began its own study, scheduled to last two years. The actual timing was a bit longer, and findings of the study were released in April 2005.

The forensic analyses revealed that the aircraft impacts severed columns in each building, which distributed the weight of the upper floors onto the remaining columns. As the fires subsequently raged, these columns weakened to the point of buckling inward and eventually collapsed.

Thus, it was not the impact, but the heat from the burning jet fuel, that heated the temperature of the buildings' steel support structures up to 1,472°F (800°C), causing them to buckle and the floors to collapse downward. (Both jets were bound for Los Angeles and had almost a full tank of fuel on board.) The initial crash neutralized sprinkler systems, allowing the spread of the fire, which was fed by caches of paper and other flammable materials in the buildings' offices.

In another forensic aspect following the incident, forensic accountants began the task of tracing the perpetrators using financial records and other materials. These investigations led to the al Qaeda terror network as the perpetrators of the attack.

see also Aircraft accident investigations; Architecture and structural analysis; DNA profiling; World Trade Center, 1993 terrorist attack.