WASHINGTON -- President Donald Trump addressed the deadliest mass shooting in US history on Monday morning from the White House, calling it an "act of pure evil."
The gun attack on a country music festival Sunday in Las Vegas killed at least 58 people and injured more than 500 others.
Continuing Coverage: Las Vegas Massacre
"My fellow Americans, we are joined together today in sadness, shock and grief," Trump said from the Diplomatic Reception Room.
The president said he had ordered American flags to be lowered to half-staff as a sign of of mourning for the victims of the shooting. He said he planned to travel to Las Vegas on Wednesday.
The massacre in Nevada is the worst domestic act of violence of Trump's presidency. He was briefed on the situation Monday morning by his chief of staff, John Kelly, and conveyed his initial condolences on Twitter.
"My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!" he wrote.
White House press secretary Sarah Sanders called the attack a "horrific tragedy" and said in a statement that the White House was "monitoring the situation closely."
As a candidate, Trump used the Orlando night club shooting -- until Monday, the worst mass shooting in American history -- as a data point in his push to temporarily ban Muslims from entering the United States.
"I said this was going to happen -- and it is only going to get worse," Trump said in a statement then.
Trump was criticized for immediately connecting the incident to radical Islamic terrorism, but later he was defiant when the shooter was identified.
"Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!" Trump wrote on Twitter in the aftermath of the Orlando shooting.
Little is known about the perpetrator of the Las Vegas attack. Authorities have identified 64-year-old Stephen Paddock as the gunman.
During the course of his presidency, Trump's predecessor Barack Obama responded to more than a dozen mass shootings. His responses ranged from angry, to emotional, to -- by the end of his term -- resigned.
On Monday, Trump was originally scheduled to speak at an event about regulatory reform, but his public remarks were canceled. Later in the day he's slated to meet with Republican governors and the prime minister of Thailand.
He still plans to travel on Tuesday to Puerto Rico to survey storm damage, the White House said.
Attorney General Jeff Sessions was meeting with the FBI on Monday morning, a Justice Department official said. Sessions also spoke briefly with Clark County Sheriff Joe Lombardo and offered support, the official said, adding that the Las Vegas shooting was the "top priority" for the day.
Although an administration official told CNN the President's trip to Puerto Rico to survey the hurricane response there, which was planned for Tuesday, was under review, Sanders said later in the morning that the visit was still on.
Vice President Mike Pence issued a statement on Twitter, saying he and his wife were praying for the victims.
First lady Melania Trump tweeted her "heart and prayers" to the victims and those affected.
Ivanka Trump, the President's eldest daughter and a top adviser, tweeted "our collective hearts are breaking for the victims and their families."
Nevada politicians also responded Monday morning to the massacre, decrying the violence and offering thanks to first responders.
"Senseless, horrifying act of violence in Las Vegas tonight," Republican Sen. Dean Heller, who is from Nevada, tweeted. "Praying for all the victims & those impacted by the tragedy."
"Thankful for police and first-responders on the scene," Heller said.
Heller said he had spoken with Nevada Gov. Brian Sandoval and state Attorney General Adam Laxalt.
Nevada's Democratic Sen. Catherine Cortez Masto offered prayers to the victims and thanks to the first responders on Twitter.
"Praying for all those affected by this senseless tragedy. Thank you to all the first responders. I will continue to monitor the situation," Cortez Masto tweeted.
Sandoval, a Republican, called the shooting a "tragic & heinous act of violence" and offered his prayers.
"A tragic & heinous act of violence has shaken the #Nevada family. Our prayers are w/ the victims & all affected by this act of cowardice," he tweeted.
Sandoval said in a statement that he would be in Las Vegas to meet with law enforcement and first responders and to console victims, their families and friends.
Members of Nevada's House delegation offered their thoughts as well.
Democratic Rep. Ruben Kihuen thanked the first responders in a tweet, and Rep. Dina Titus, also a Democrat, said in a statement that her "heart aches for the victims, their loved ones and our community."
As the morning wore on, politicians from around the country extended their sympathies to the victims of the shooting and thanks to first responders.
Florida Republican Sen. Marco Rubio, whose state suffered a massive shooting in an Orlando nightclub last year, tweeted his prayers, and Connecticut Democratic Sen. Chris Murphy, who has kept up a drumbeat for gun control in response to the Sandy Hook Elementary School shooting in his state in 2012, tweeted, "Not again."
The state's senior senator, Democrat Richard Blumenthal, said, "The nation's conscience must be galvanized."
New York Democratic Sen. Kirsten Gillibrand went further than many of her counterparts, saying her heart was with the victims, but "thoughts and prayers" were not enough.
"We must act to prevent this from happening again," Gillibrand tweeted.
Former Arizona Democratic Rep. Gabby Giffords, herself a victim of gun violence along with several others after an assassination attempt in 2011, said on Twitter that "no person should endure the horror Las Vegas experienced last night."
House Majority Leader Kevin McCarthy called the shooting "a senseless tragedy."
The chamber's minority leader, Nancy Pelosi wrote on Twitter that she was "horrified and heartbroken."
Former President Barack Obama tweeted: "Michelle & I are praying for the victims in Las Vegas. Our thoughts are with their families & everyone enduring another senseless tragedy."
Former President Bill Clinton wrote, "Thinking of the victims and responders in Las Vegas. This should be unimaginable in America."
Foreign leaders sent their condolences on the shooting Monday morning. United Kingdom Prime Minister Theresa May responded on Twitter, as did London Mayor Sadiq Khan.
"PM -- The UK's thoughts are with the victims and the emergency services responding to the appalling attack in Las Vegas," May tweeted.
Khan tweeted, "A deeply sad day for the city of Las Vegas. London sends our condolences to the victims and their families."
Canadian Prime Minister Justin Trudeau said, "Words fail this morning. The friendship & support of Canadians is with the victims in Las Vegas & the people of the US."