How did the FAA find out that flight 93 was hijacked?

A minute later, the transponder was turned off, but the Cleveland controller continued to monitor the flight on primary radar. The Herndon Center relayed information on Flight 93 to FAA headquarters. Joseph DeLuca called his father at 09:43:03 from row 26 to inform him the flight had been hijacked.

Explanation:

A minute later, the transponder was turned off, but the Cleveland controller continued to monitor the flight on primary radar. The Herndon Center relayed information on Flight 93 to FAA headquarters. Joseph DeLuca called his father at 09:43:03 from row 26 to inform him the flight had been hijacked.

Explanation:

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