After dropping its 2013 scrutiny of Mateen, the bureau is under fire for not doing more to prevent the worst mass shooting in modern US history. Yet on other occasions in recent months, the FBI has been criticised for its handling of domestic terror probes, including the use of controversial sting operations.

"They're in a very hard position right now," says David Gomez, who retired in 2011 after a 28-year FBI career and who once headed the Joint Terrorism Task Force in Washington State. "They're being criticised for doing what they're legally entitled to do."

Orlando marks the third attack — including the 2013 Boston Marathon bombings and the shootings last year at a Garland, Texas, exhibit of cartoons of the Prophet Mohammed — involving an individual who had been interviewed by the FBI.