The devastation was "the most significant challenge that the National Communications System had ever seen," according to Brenton Greene, deputy manager at the National Communications System (NCS), which is responsible for all the major telecommunications networks that have national security significance.
Mobile telephone circuits were immediately overloaded. One national mobile operator reported that network call traffic surged to 400% of its normal level in the two hours after the first incident.
The collapse of the towers sent a massive steel beam slicing through a bundle of fibre cables buried eight feet below ground destroying more than four million high-speed access lines. The beam also ruptured water pipes that filled underground switching vaults with more than 10 million gallons of water.
The damage knocked out 1.5 million circuits that served the financial district, said Greene.
The Verizon facility housed enough equipment to make it the "most communications-intensive area in the US," said Bruce Fleming, divisional technology officer at the communications company Sprint.
Telecoms companies WorldCom and AT&T also suffered severe damage.
Restoring the Verizon backbone required an army of technicians. Primary power had been lost, and backup power, which was running on diesel fuel generators, began to fade quickly.
Lucent Technologies, one of Verizon's main systems providers, rushed a 100,000-line switch to the scene to replace one major switch that had been sent crashing through the window of the Verizon building. The company also put all of its customer requirements on hold and made its entire inventory available to rescue services, said Greg Butler, a Verizon vice-president who coordinated the response efforts.
In addition to the damage incurred by Verizon, at least 139 fibre rings in surrounding buildings and 26 building-specific fibre rings failed, said Dick Price, vice-president of field operations at WorldCom.
"From a macro level, our national security should be a major part of our telecommunications policy," said Fleming. "If it can happen, it hasn't happened yet."