Question: The terrorist attacks against the United States on 11 September 2001 left over 3000 victims in three different locations: the Pentagon in Washington, DC, a fi eld near Shanksville (Somerset County), Pennsylvania, and the twin towers of the World Trade Center (WTC) in New York City. DNA samples from the Pentagon and Pennsylvania sites were processed by the Armed Forces DNA Identifi cation Laboratory (AFDIL) while the WTC work was performed by the New York City Offi ce of Chief Medical Examiner Department of Forensic Biology, the New York State Police, and a number of contract laboratories and consultants. The DNA identification efforts for the WTC victims have become arguably the world's largest forensic case to date. More than 19,917 pieces of human remains were collected from a pile of rubble weighing over a million tons and extending more than 70 feet (21 m) in height following the crushing collapse of the Twin Towers.

The initial removal and sorting of human remains took place between September 2001 and May 2002. However, the primary DNA identification efforts went on for more than 3 years - almost 2.5 years after the last piece of debris had been removed from the WTC site. As of late 2008, more than 1600 victims of the 2749 present when the Twin Towers collapsed had been identi- fied. Without the capabilities of DNA testing, there would have been only a fraction of the victims identified based on other modalities such as fingerprints and dental records. Biological samples recovered from the WTC site had been subjected to extreme pressures with the building collapse and then subterranean fi res of 1500 ° F (815 ° C) or more for the 3 months following the terrorist attack.

The jet fuel from both planes that rammed the WTC towers burned intensely enough to melt the steel support beams and bring down the buildings. Thus, human remains in this pressure cooker were often comingled, very fragmented, and in many cases likely vaporized. Severalinnovations came out of the 9/11 tragedy. These included new extraction methods from bone, reduced size amplicons or miniSTRs, panels of single nucleotide polymorphisms (SNPs), and high-throughput mitochondrial DNA sequencing. In addition, new software was developed to aid in matching reference samples and recovered remains as well as associating remains with the same DNA profi le. Oneof the largest challenges from this investigation was review of the massive amounts of data produced by contracting laboratories.

More than 52,528 STR profi les, 16,938 SNP profi les, and 31,155 mtDNA sequences were generated in an effort to identify the 2749 victims of the World Trade Center collapse based on 19,917 recovered remains - truly a heroic effort. Most of the data from the recovered remains contained only partial DNA profiles, making it even more difficult to sort through and piece together sufficient information to make a reliable identification. While we hope to never see the likes of another 11 September 2001 terrorist attack, forensic DNA typing laboratories should be prepared to aid in victim identification efforts in future mass fatality incidents.