Declaring it a "very, very sad day" for himself and the nation, Donald Trump has visited a Las Vegas hospital, in a city reeling from the worst mass killing in modern US history.
But the US President avoided answering questions on whether or not the country has a problem with gun violence.
- Trump visits hospitals and first responders
- Trump says investigators still determining motive of shooter, Stephen Craig Paddock
- Paddock's girlfriend Marilou Danley questioned
Mr Trump met with survivors and law enforcement officials in the aftermath of a gunman killing 59 people, including himself, at a concert.
Air Force One landed at the airport near the famed Las Vegas strip on a bright, sunny morning just days after the gunman on the 32nd floor of a hotel and casino opened fire on people at an outdoor country music festival below.
"America is truly a nation in mourning," he said from a press conference at the Las Vegas Metropolitan Police headquarters.
"Our souls are stricken with grief for every American who lost a husband or a wife, a mother or a father, a son or a daughter. We know that your sorrow feels endless. We stand together to help you carry your pain. You are not alone. We will never leave your side."
He said authorities were "learning a lot more" about the shooter, Stephen Craig Paddock, and that more details would be "announced at an appropriate time".
"It's a very, very sad day for me personally," he said.
His comments came as Paddock's girlfriend Marilou Danley, accompanied by her lawyer, was interviewed at the FBI's office in Los Angeles, according to a law enforcement official who was not authorised to discuss the case publicly and spoke on condition of anonymity to Reuters and AP.
In a statement read by her lawyer, Ms Danley said she returned to the US because she was aware the FBI and Las Vegas police wanted to interview her, "and I want to talk to them".
"I will cooperate fully with their investigation. Anything I can do to help ease suffering and help in any way, I will do," she said.
"Please respect my privacy and my family's privacy. Thank you."
Ms Danley, 62, who has been called a "person of interest" by investigators, was met by federal agents on Tuesday night when she arrived at the Los Angeles airport from her native Philippines after more than two weeks abroad.
Her two sisters living in Australia said Ms Danley, who is a mother and grandmother, had no idea Paddock had a cache of weapons or that he intended to use them to gun down hundreds of people.
Her brother, Reynaldo Bustos, told ABC America from his home outside Manila that Danley had assured him: "Do not panic. I have a clean conscience."
Investigators are busy reconstructing Paddock's life, behaviour and the people he encountered in the weeks leading up to the shooting, FBI deputy director Andrew McCabe said.
That includes examining his computer and mobile phone.
Speaking from a hospital where victims were being treated, Mr Trump said investigators were still searching for Paddock's motivation for the attack.
"We're looking very, very hard," he said after calling Paddock "a very demented person".
Trump avoids gun control questions
The President praised the bravery of survivors who risked their lives to help other victims as bullets rained down.
Some of the patients the President and First Lady Melania Trump met with "were very, very badly wounded," Mr Trump said.
"They were badly wounded because they refused to leave. They wanted to help others because they saw people going down all over."
The Trumps' motorcade passed the Mandalay Bay hotel, where Paddock fired at least two firearms from the window, during the drive to the hospital.
Paddock killed himself as police closed in.
During the visit, Mr Trump deflected a question about whether the United States has a problem with gun violence.
"We're not going to talk about that today," he said.
Republicans who control Congress have made clear they have no intention of taking up gun control measures, in the shooting's aftermath, such as tightening restrictions on semi-automatic weapons and high-capacity magazines.
On Tuesday, Mr Trump appeared somewhat open to having a debate on guns, but not anytime soon.
"At some point, perhaps, that will come," he told reporters. "But that's not for now."