After slamming the president's immigration policy, Donald Trump said Hillary Clinton "wants to allow radical islamic terrorists to enter our country... they enslave women and they murder our gays."
A journalist walked off TV when the anchor wouldn't acknowledge it was an anti-LGBT attack.
James Comey told reporters the killer had been investigated twice since 2013, but there was insufficient evidence to charge the 29-year-old security guard with a crime.
The presumptive Democratic presidential nominee spoke about the massacre in Florida at a rally in Cleveland, where Republicans meet to choose their nominee next month.
Friends and allies of LGBT folks were also in Omar Mateen's line of fire.
While investigators pore over about 100 leads, the FBI, Florida Department of Law Enforcement and Orlando Police said at a news conference notifying next of kin is their top priority.
Seddique Mir Mateen spoke to reporters about his son, Omar Mateen, and he clarified his remarks following the massacre at Pulse early Sunday.