WASHINGTON -- President Donald Trump stuck to a sombre script Monday after at least 59 people were shot dead in Las Vegas, condemning the largest mass shooting in modern U.S. history as an "act of pure evil" and declaring the nation would unite behind the survivors. He refused to get into a new debate over gun control.

Faced with the sad and familiar ritual of a president offering consolation after horrific violence, Trump spoke slowly and carefully from the White House Diplomatic Room, focusing not on the identity or possible motive of the shooter but on the nation's efforts to heal.

The remarks were the prelude to a two-part test of whether Trump, a leader who excels at political provocation, can also deliver solace. Trump is slated to visit Puerto Rico Tuesday to survey devastation from Hurricane Maria, before going to Las Vegas Wednesday to meet with families of the dead.

"Our unity cannot be shattered by evil, our bonds cannot be broken by violence," the president said. "We call upon the bonds that unite us: our faith, our family, and our shared values. We call upon the bonds of citizenship, the ties of community, and the comfort of our common humanity."

Trump spoke hours after a gunman on the 32nd floor of a Vegas Strip casino opened fire on people at an outdoor country music festival below. The gunman, 64-year-old Stephen Craig Paddock, killed at least 59 people as tens of thousands of concertgoers screamed and ran for their lives. More than 500 people were hurt.

In a measured statement that was revised by aides until moments before he spoke, Trump did not describe the gunman in any way or suggest what might have been behind his actions. He praised the first responders who he said prevented further loss of life and said he would visit Las Vegas on Wednesday. He offered condolences to the families of those killed, saying, "We cannot fathom their pain. We cannot imagine their loss."

Trump, who owns a hotel in Las Vegas, told associates that he feels deep ties to the city. He said in public that his visit would be "a very, very sad moment for me ... for everybody no matter where you are, no matter what your thought process."

The president was informed of the shooting early Monday by chief of staff John Kelly, according to White House press secretary Sarah Huckabee Sanders. He received periodic briefings from Kelly and homeland security adviser Tom Bossert while also monitoring the coverage on cable news. On Twitter, he avoided the kind of inflammatory statements he has issued after some previous tragedies, such as last year's Orlando nightclub shooting, instead offering "warmest condolences" to victims and their families.

"There's a difference between being a candidate and being the president," Sanders said when asked to explain the change in tone.

Trump's visit to Las Vegas will come a day after he travels to hurricane-ravaged Puerto Rico, where many have accused the federal government of an inadequate response. Aides have worried about the response he will receive there, as Trump spent the weekend attacking the mayor of San Juan while consistently painting a rosier picture of the recovery effort than what has been described by those on the ground.

The back-to-back trips to Puerto Rico and Nevada will also place Trump into the role of consoler-in-chief -- a considerable change from his usual persona as a firebrand feeding off division.

The weight of the violence hung heavy at the White House on Monday. Conversations were hushed throughout the West Wing, and Sanders choked up during the press briefing as she read news accounts of heroism displayed at the scene.

Like presidents before him, Trump marked the events by ordering the American flags at all public buildings across the nation flown at half-staff, and he observed a moment of silence on the White House lawn. Unlike his predecessor, Barack Obama, who responded to mass shootings by calling for stricter gun control measures, Trump made no mention of firearms restrictions.

Sanders repeatedly rebuffed questions about whether laws need to be stricter, saying the day after the mass shooting was not the "time and place for a political debate."

But the familiar discourse did ignite Monday, once again breaking along party lines.

Democratic Sen. Chris Murphy of Connecticut said it was time for Congress to end the inaction that followed other major shootings, including the one in Orlando and the 2012 school shooting in his home state. House Minority Leader Nancy Pelosi called for the creation of a committee on gun violence and increased background checks. Rep. Seth Moulton, D-Mass., said he would not participate in a moment of silence for those lost because it "becomes an excuse for inaction."

Former Rep. Gabrielle Giffords, who was critically wounded in an assassination attempt six years ago, appeared at the Capitol and said she knew "this feeling of heartbreak and horror too well." She called for Congress to pass stricter gun control laws, turning to the Capitol building, raising her fist and saying, "The nation is counting on you."

But Republicans largely avoided the subject. Both House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell offered sympathetic tweets for those who lost their lives but made no mention of gun policy.

Before he was a candidate, Trump at one point favoured some gun restrictions. He has more recently cast himself as an ardent protector of the Second Amendment. Though Sanders did not dismiss a question about the potential for a bipartisan effort on guns, raising the possibility that Trump might be willing to cross the aisle and work with Democratic leaders, the president has proclaimed that if more "good guys" were armed with firearms, there would be fewer gun tragedies.

He told the National Rifle Association this year that it had a "true friend" in the White House, and he signed a resolution passed by the GOP-led Congress blocking an Obama-era rule designed to keep guns out of the hands of certain mentally disabled people.

This was not the first time Trump offered a sombre response to a tragedy: In June, he was measured in his response to a shooting at a congressional baseball practice that wounded five, including Republican Rep. Steve Scalise. But Trump has drawn criticism for more inflammatory and self-referential reactions to other acts of violence.

After the Orlando shootings at a gay nightclub that left 49 dead, he tweeted, "Appreciate the congrats for being right on radical Islamic terrorism." In the wake of a deadly terror attack in London in June, he went after the city's mayor on Twitter, suggesting he wasn't taking the attacks seriously enough.

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Lemire reported from New York. Associated Press writers Jill Colvin, Matthew Daly and Kevin Freking contributed reporting.