U.S. President Barack Obama said Tuesday that the country's biggest mass shooting in history was carried out by an "angry, disturbed, unstable young man who became radicalized."
Obama reiterated that investigators "do not have any information to indicate that any foreign terrorist group directed the attack" that an American-born Muslim, 29-year-old Omar Saddiqui Mateen, carried out early Sunday at a gay nightclub in Orlando, Florida.
In a three-hour rampage, Mateen killed 49 people and wounded another 53, six of them critically.
The president, speaking after a broad national security review of the Orlando rampage and U.S. efforts to defeat Islamic State terrorists in the Middle East, said Mateen "took in extremist information and propaganda over the internet."
Obama said U.S. law enforcement officials "are doing everything to stop these kinds of attacks," but are "sobered" by the difficulty in trying to detect "these lone actors" beforehand.
Gun control laws
Earlier, U.S. Homeland Security chief Jeh Johnson called for "meaningful, responsible" gun control laws in the aftermath of the Orlando mass shooting, while authorities continued to collect more information about Mateen's life.
In a round of interviews, Johnson told television news shows that tighter gun restrictions, long a contentious issue in the United States, are "critical to public safety."
"We have to face the fact that meaningful, responsible gun control has to be part of homeland security as well, given the prospect of homegrown, home-born violent extremism in this country," Johnson said on CBS This Morning. "It's something that I think the American public and the Congress has to face and has to address."
Patrons at the Pulse nightclub said they had frequently seen Mateen there over the past several years. Others said they encountered Mateen, twice married, once divorced, and with a three-year-old son, on the gay chat and dating app "Jack'd."
One man, Ty Smith, said he remembered seeing Mateen at Pulse at least a dozen times.
"Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent," Smith said.
Seen as a 'regular'
Another man, Jim Van Horn, 71, said Mateen was a "regular" at Pulse.
"He was trying to pick up people. Men," Van Horn told The Associated Press.
Van Horn and others familiar with the gay club scene in Orlando say that Mateen may have been conflicted about his sexuality and acted out of anti-gay hatred in carrying out the rampage.
Mateen's father, Seddique Mateen, has told reporters his son became visibly upset after seeing two men kissing on a recent visit to Miami, another Florida city in the southeastern U.S.
Law enforcement authorities say that twice in recent years they investigated Mateen's connections with terrorist groups, but concluded no charges were warranted and dropped further inquiries.
Now, the Federal Bureau of Investigation, the country's top law enforcement agency, is taking a look back to see if signs of Mateen's radicalization were missed, even as it collects volumes of information about his recent activities leading up to his assault on the club.
Mateen's shooting spree bore an ominous connection to other tragic mass killings in the United States carried by out by attackers using legally available AR-15 semi-automatic rifles.
Mateen used the weapon to shoot a huge number of rounds at late-night revelers at the club, many of them of attracted by Latin-themed dance music last Saturday and into the early hours of Sunday when the attack occurred.
Mateen legally bought the powerful assault weapon "in the past few days," according to authorities.
The AR-15 allows shooters to fire as many rounds as fast as they can pull the trigger. The AR-15 was prohibited for sale in the United States in 1994, but the ban was lifted in 2004 and has not been renewed.
The powerful gun lobby in the United States, led chiefly by the National Rifle Association, has fought off numerous gun control efforts in the country, even after deadly attacks that briefly caught the attention of the nation.
The AR-15 was used by killers responsible for two of the deadliest incidents in recent U.S. history, a 2012 assault on an elementary school that killed 26 people, most of them schoolchildren, and an attack the same year at a movie theater.
WATCH: President Barack Obama on Orlando suspect
Authorities in Orlando said they are working on a hundred leads to try to determine if Mateen had help in carrying out the attack.
"There may be prosecutions down the road," FBI agent Paul Wysopal told reporters.
Authorities say that during his attack on the club, Mateen called the Orlando emergency line and vowed fealty to the Islamic State group and its leader, Abu Bakr al-Baghdadi, and also referenced the 2013 Boston Marathon bombing that killed three people.
But the shooter's father, Seddique Mateen, denied his son's actions had anything to do with Islam.
'He had hatred'
"I don't know what happened and I didn't know he had hatred. He went to a gay club for men and women and shot 50 of them with a gun," Seddique Mateen told reporters Monday.
He described his son as "a good son and an educated person." But he later went on to say, "God will punish those involved in homosexuality," saying it is "not an issue that humans should deal with."
The White House said Obama will travel Thursday to Orlando "to pay his respects to victims' families, and to stand in solidarity with the community as they embark on their recovery."
WATCH: President Barack Obama on extremist views of Orlando suspect