The people in the twin towers had half a million tonnes of steel and concrete crashing down on top of them. This caused so much destruction that many of the bodies were completely crushed to dust and pulverised / destroyed, so much so that in many cases there wasn't even a scrap of skin or bone or blood to be identified. About half of the victims (i think) have been identified from DNA testing on scraps of bones or blood, etc., but many of the victims did not even leave enough of a trace for that. Therefore the identification of hundreds of the victims has to rely on evidence of where they were, where and when they were seen, and where they were expected to be at the time. There were many regular workers who would have been in the building anyway, and known to be so. There were also many casual visitors. Also, there were a number of illegal or unofficial workers in the restaurants and shops etc. who may have been illegal immigrants / migrant labourers, and for whom there is no known or traceable relative. A number of people who were believed to be victims were found alive days or weeks later, some of whom had fraudulently used the event to claim payouts on life-insurance policies. Similarly, a number of people simply disappeared and it will only ever be as matter of supposition or guesswork as to whether they were in the building at the time. Initially, many victims were counted or listed twice under slightly different names or with different addresses. For these reasons, the "official death toll is constantly being revised. It will never be known exactly how many victims were killed, or exactly who they all were. As such it was such as huge event that it was a bit like an earthquake in terms of trying to find or identify all the victims.