President Trump in an address to the nation Thursday called the Florida school shooting that left 17 people dead and another 15 wounded a moment of "hatred and evil."

"To every parent, teacher and child who is hurting so badly, we are here for you whatever you need," Trump said. "Whatever we can do to ease your pain, we are all joined together as one American family. Your suffering is our burden."

Trump's address came about two hours after the president issued a proclamation honoring the victims of one of the worst school shootings in U.S. history, ordering the American flag to be flown at half-staff at the White House and all public buildings and grounds.

"Our Nation grieves with those who have lost loved ones in the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida. As a mark of solemn respect for the victims of the terrible act of violence perpetrated on February 14, 2018, by the authority vested in me as President of the United States by the Constitution and the laws of the United States of America, I hereby order that the flag of the United States shall be flown at half-staff at the White House and upon all public buildings and grounds, at all military posts and naval stations, and on all naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions until sunset, February 19, 2018. I also direct that the flag shall be flown at half-staff for the same length of time at all United States embassies, legations, consular offices, and other facilities abroad, including all military facilities and naval vessels and stations."

A 19-year-old former student with a history of menacing social media posts was booked into Broward County Jail Thursday morning on charges of premeditated murder in the killing of the 17 students and faculty at Marjory Stoneman Douglas High School in Parkland, Fla. near Fort Lauderdale.

Earlier Thursday, Trump tweeted that there were "so many signs" that the shooter was "a big problem."

Nikolas Cruz, who was expelled last year for fighting, returned to the school Wednesday with a gas mask, smoke grenades and multiple magazines of ammunition and a semi-automatic weapon.

The shooting drew an outcry among Democratic lawmakers on Capitol Hill that it was time for additional gun control measures, such as a ban on assault rifles and expanded background checks, few of which are expected to get any traction in the GOP-controlled Congress.

"When is this going to stop?" tweeted Sen. Bill Nelson, D-Fla.

Attorney General Jeff Sessions, addressing the Major County Sheriffs of America conference in Washington Thursday morning,called the shooting "painful."

"It's certainly a tragic event an we're going to work on it in may ways to do something about it," he told the sheriffs. "It cannot be denied that something dangerous and unhealthy is happening in our country. ... We've got to confront the problem. There's no doubt about it."

Contributing: Doug Stanglin, USA TODAY