Fortune commissioned a team of reporters to reach out to the companies who were World Trade Center tenants during the attacks on Sept. 11, 2001, and find out where they are 10 years later. Numbers were provided by company representatives or employees who worked in the towers, with some additional data coming from Port Authority records, wire reports and other sources. Contractors and consultants are generally not listed, though some firms did include them in their count. Several firms were only able to provide an estimate for total number of employees, though exact figures were used when possible.
A few companies, including Dun Bradstreet and Washington Group (now part of URS Corporation), declined to participate or provide information and were not included.
Because of 10 years of mergers and acquisitions, it was difficult to find precise figures for several of the major businesses in the towers. The 2011 location refers to the closest possible equivalent of the divisions and branches that were housed in the World Trade Center. Where this information was not available because those offices no longer existed in their 2001 incarnations, Fortune used the headquarters of the parent company to represent current location.
In most cases, the current employee count refers only to the specific office shown on the map, and does not include other offices around the country or the world. The size of those offices and divisions does not necessarily reflect the growth or performance of the firm.
*The restaurant Windows on the World was destroyed in the attack and was not rebuilt.