US authorities say they have found no direct links between Islamic State and the gunman who killed 49 people at a gay nightclub in Orlando.
Police have described him as a homegrown extremist who was inspired by radical Islamist groups.
The FBI and other agencies were still looking at evidence inside the club and on the closed-off streets around the Pulse nightclub, where New York-born Omar Mateen perpetrated the worst mass shooting in modern US history.
Mateen, 29, the son of Afghan immigrants, was shot and killed by police who stormed the club with armoured cars after a three-hour siege.
"So far, we see no indication that this was a plot directed from outside the United States and we see no indication that he was part of any kind of network," FBI Director James Comey said in Washington. "We're highly confident this killer was radicalised at least in some part through the internet."
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During phone calls with authorities in the middle of the rampage, Mateen mentioned support for the leader of Islamic State, the Boston Marathon attackers and a Florida man who became an al Nusra Front suicide bomber in Syria, Comey said.
Al Nusra is an al Qaeda offshoot at odds with Islamic State.
Law enforcement officials searched for clues as to whether anyone had worked with Mateen on the attack, said Lee Bentley, the US attorney for the middle district of Florida.
"There is an investigation of other persons. We are working as diligently as we can on that," Bentley told a news conference. "If anyone else was involved in this crime, they will be prosecuted."
Officials said they believed there had been no other attackers and had no evidence of a threat to the public.
Mateen's father said his son was not radicalised but indicated the gunman had strong anti-gay feelings. His ex-wife described him as mentally unstable and violent toward her.
Islamic State reiterated a claim of responsibility, but that did not mean it had directed the attack because it offered nothing to indicate coordination with the gunman.
The carnage early on Sunday occurred in the heart of Orlando, about 25 km northeast of the Walt Disney World Resort.
Mateen was an armed guard at a gated retirement community, and had worked for the global security firm G4S for nine years.
He had cleared two company background screenings, the latest in 2013, according to G4S..
Authorities said on Sunday that Mateen had been twice questioned by Federal Bureau of Investigation agents in 2013 and 2014 after making comments to co-workers about supporting militant groups, but neither interview led to evidence of criminal activity.
Mateen visited Saudi Arabia in 2011 and 2012 for religious pilgrimages, a government spokesman said on Monday.
