RTI International, 230 W Monroe St, Suite 2100, Chicago, IL 60606, USA. jmurphy@rti.org
I applied the capture-recapture method to estimate the World Trade Center tower population at the time of the September 11, 2001, terrorist attacks. Available lists helped identify 8965 survivors and 2152 confirmed casualties. The capture-recapture model suggested that an additional 4435 survivors were present, putting the total count of all present at 15,552 (95% confidence interval=15,216, 15,897). An accurate estimate represents the potential number at risk for trauma as a result of direct exposure to the events of the day.