The latest media reports also said IS has claimed responsibility for the nightclub attack.
The Federal Bureau of Investigation (FBI) said Mateen was once investigated for possible terrorist ties.
Speaking at a press conference, FBI official Ronald Hopper said the shooter legally purchased at least two firearms recently.
However, federal officials said so far they had seen no immediate evidence linking the terror group based in Syria and Iraq to the shooting spree in Orlando.
President Obama had ordered U.S. flags at half-staff.
Obama's statement on Sunday marked at least the 20th time he had addressed the nation on the topic of mass shooting during his presidency.
Calling the Florida shooting spree "the most deadly shooting in American history," Obama again reminded the country of the sober reality that "how easy it is for someone to get their hands on a weapon" that could launch mass killings.
Following the 2012 school mass shootings in Newtown, Connecticut, which claimed 26 lives, including 20 children, the Obama administration initiated but failed to push stronger gun control laws.
The laws, whose sections included expanded background checks and bans on assault weapons, were stymied in Congress after staunch opposition from Republican lawmakers and gun-rights lobby groups.
During his presidency, Obama has been confronted with more than a dozen of high-profile mass shootings, and in an interview last year he called the failure to reform U.S. gun laws "one of the greatest frustrations" of his presidency.
"If you ask me where has been the one area where I feel that I've been most frustrated and most stymied, it is the fact that the United States of America is the one advanced nation on Earth in which we do not have sufficient common-sense gun safety laws, even in the face of repeated mass killings," Obama told BBC in an interview in July, 2015.
