Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) on Monday ordered flags at the U.S. Capitol complex to be lowered to half-staff in honor of the victims of the Las Vegas mass shooting.

More than 50 people were killed and more than 500 were injured at an outdoor music festival on the Las Vegas Strip Sunday night in the deadliest shooting in U.S. history.

“America woke up this morning to heartbreaking news. This evil tragedy horrifies us all,” Ryan said in a statement on Monday. “To the people of Las Vegas and to the families of the victims, we are with you during this time. The whole country stands united in our shock, in our condolences, and in our prayers.”

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Police say the suspected gunman, Stephen Paddock, 64, of Mesquite, Nevada, fired at concert-goers below from the 32nd floor of Mandalay Bay Resort and Casino.

The shooting came just three days after a key member of the GOP leadership team, Majority Whip Steve Scalise (R-La.), returned to the Capitol for the first time since he was nearly killed in a mass shooting. Scalise in June was among four people hit by a gunman, who was fatally shot by police, at a Republican baseball practice in Virginia.

Because of the scale of Sunday night’s tragedy, lawmakers on Capitol Hill and candidates across the country decided to cancel previously scheduled events on Monday.

House Minority Whip Steny Hoyer (D-Md.) and other Democrats had been set to hold a news conference urging action to help hurricane victims in Puerto Rico and the U.S. Virgin Islands, but that event was postponed. House Armed Services Chairman Mac Thornberry (R-Texas) also delayed a briefing with reporters.

Nevada Attorney General Adam Laxalt, a Republican, had been set to announce his bid for governor on Monday, the start of a weeklong campaign tour through the state. But he cancelled all political events after the shooting.

House Majority Leader Kevin McCarthy (R-Calif.), the No. 2 Republican in leadership, said several off-duty officers from his hometown of Bakersfield were attending the concert. One of them was injured.

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“Our nation woke up this morning to tragedy on an unimaginable scale. These are times when we turn to God trying to understand how such inhuman acts can happen,” McCarthy said in a statement. “Our nation prays for those who died and for those still fighting to live. Las Vegas is a place that people from around the country and the world go to visit.

“Communities across America will feel the pain of this violence personally—including my hometown of Bakersfield.”

House Minority Leader Nancy Pelosi (D-Calif.) also paid tribute to the “innocent men and women whose lives were cruelly cut short by this tragedy.”

"It is particularly despicable that this attack was carried out during a concert, transforming an evening of music and celebration into a heartbreaking and horrific night of violence,” Pelosi said. “Our nation will never allow fear and violence to bow us or break our spirits."

—This report was updated at 11:21 a.m.