Yes, the F.B.I. could have stopped 9/11. It had a warrant to establish surveillance of everyone connected to Al Qaeda in America. It could follow them, tap their phones, clone their computers, read their e-mails, and subpoena their medical, bank, and credit-card records.
While we don’t know the investigators’ other methods, the FBI has an impressive arsenal of tools to track Broadwell’s digital footprints — all without a warrant.