Washington: IS claimed responsibility on Sunday for the worst mass shooting in US history, but US officials said they had seen no immediate evidence linking the militant group to the massacre in Orlando, Florida.
IS' claim was carried by Amaq, the organization's news agency.
At least 50 people were killed and 53 others were wounded in the Pulse nightclub before the suspected gunman was shot to death by police.
The suspected shooter was identified by authorities as Omar Mateen, a Florida resident who a senior FBI official said might have had leanings toward IS.
The FBI official cautioned, however, that proving the suspected link to radical Islamism required further investigation.
Two US officials familiar with the investigation into the massacre said that no evidence had yet been found showing a direct link between the massacre and IS or any other militant group.
The officials, who spoke on condition of anonymity, also said they had yet to see any direct contacts between any extremist group and the suspect.
Speaking at the White House, US President Barack Obama called the attack "an act of terror" and an "act of hate," and said the FBI would "spare no effort" to determine whether the suspect had been inspired by any extremist group.
The two officials familiar with the investigation said a leading theory was that the suspect somehow was inspired by extremist militants.
Federal authorities believe the shooter was Mateen, the U.S.-born son of Afghan immigrants, he said.
US Representative Adam Schiff of California, the senior Democrat on the House Intelligence Committee, said in a statement issued after a briefing on the massacre that several factors indicated the attack was an IS inspired "act of terrorism."