On a call with other members of the industry, as the problem grew worse, N.Y.S.E. employees expressed “panic” and a recognition that they had “lost control of the system,” according to a person on the call, who spoke on the condition of anonymity.

Not long after that call, at 11:32 a.m., the exchange announced that it was shutting down all trading, and said on its website that “additional information will follow as soon as possible.”

On the floor, traders said the exchange gave them no warning before making the move.

“I was about to order my lunch and when I turned around the screens all went blank,” said Peter P. Costa, a trader on the floor with Empire Executions.

In the silence that followed, traders on the floor, looking for information, flocked to a ramped area of the huge complex on Wall Street where employees of the exchange are located. The traders were told that the exchange had been updating software before the trading day began, and that the software had contained a bug that could not be fixed without shutting down the system.

The exchange’s employees on the floor proceeded to manually cancel around 700,000 trades that were in the system when it was shut down, according to Mr. Costa. Once they had finished, the exchange rebooted its system, a process that took around 45 minutes, Mr. Costa said.

Trading ultimately resumed around 3:10 p.m., less than an hour before the closing bell.

Several hours later, the exchange put out a statement with few details on what happened, other than to attribute blame to a “configuration issue.”