From FDNY's Battalion Chief John J. Salka, Jr. "Initial Report on FDNY's Response to the World Trade Center Attacks":
When the first plane crashed the incident was already a "gigantic, once in a lifetime operation." However, the already enormous operation was instantly multiplied by the second plane attack and the ensuing collapse of each tower.
Massive fire was visible in the towers, burning throughout five-floor sections in some areas and in locations as far as 20 floors below the places of impact.
The fire came down with the collapses and created secondary fires all over the surrounding area in buildings and cars. The most involved was the 47-story building next to the WTC, of which 35 floors were on fire.
Because so many water mains had been destroyed, two fire boats were vital in pumping enough water from the Hudson River to supply the firefighting operations at Ground Zero.