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By Adam Edelman

President Donald Trump on Thursday said there were "many signs" the suspected shooter in the horrific attack at a high school in Parkland, Florida, was mentally disturbed and that his classmates should have reported him to authorities.

"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior," Trump tweeted, referring to 19-year-old Nikolas Cruz, who was charged Thursday with 17 counts of pre-meditated murder. "Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

Later Thursday, in a nationally televised speech, Trump announced he would visit Parkland, Fla., and that "our entire nation, with one heavy heart, is praying for the victims and their families."

Trump made no reference to guns or the AR-15 believed to have been used in the attack in his tweets or in his speech.

At least 17 people were killed when a teen opened fire with a semiautomatic rifle at the high school on Wednesday afternoon, officials said. Fourteen others were wounded, five of whom suffering life-threatening injuries, hospital officials said.

Parkland Mayor Christine Hunschofsky responded to Trump's tweet just moments after he posted it, when it was read to her during a live interview on CNN, saying, "If a solution was simple for these things, we would have found one already."

Trump tweeted twice on Wednesday about the school shooting, after news of it broke, but did not make any on-camera remarks about the attack. The White House also canceled its daily press briefing Wednesday after the shooting, although on Thursday morning, the White House released a "proclamation honoring the victims" of the shooting.

"Our Nation grieves with those who have lost loved ones in the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida," Trump said in the statement. He also ordered flags be flown at half-staff.

A senior administration official also said White House advisers were discussing a possible visit by Trump to the area during his previously scheduled trip to his Mar-a-Lago estate this weekend.

Marjory Stoneman Douglas High School, where the attack occurred, is about 40 miles from Mar-a-Lago.

Meanwhile, Sen. Marco Rubio (R-FL), in an interview Thursday morning with Fox News Channel’s "Fox and Friends" called the suspect a "deeply disturbed person" and "someone that people knew was a danger."

"You know, someone that in the school there was this running joke, it’s not a joke anymore obviously, but a running joke that he was gonna come back one day and hurt a lot of people," he said.

Attorney General Jeff Sessions, during a previously scheduled speech to a group of sheriffs in Washington, D.C., admitted Thursday that "it cannot be denied that something dangerous and unhealthy is happening."

"It is too often the case that the perpetrators of these terrible attacks had given of signals in advance," he said. "Perhaps we haven’t been effective enough in intervening immediately. I suspect we’ve seen that again in this case...we cannot arrest everybody that somebody thinks is dangerous."

Trump has previously blamed mass shootings on mental health issues. In November, he said the mass shooting at a church in Sutherland Springs, Texas, "isn't a guns situation" but instead "a mental health problem at the highest level."

Just weeks into his presidency, however, Trump signed a bill rolling back a regulation that had made it harder for people with mental illnesses to buy firearms.

Trump has previously directed blame at the neighbors of other mass shooters. Following the December 2015 shooting in San Bernardino, Calif., he repeatedly claimed, without providing any evidence, that the neighbors of shooters Syed Farook and his wife, Tashmin Malik, knew of the attack.

"Many people saw the bombs all over the apartment (in San Bernardino), Muslims have to report the problems when they see them," Trump said during a presidential debate.

Trump struck a more solemn tone in addressing the nation after the Las Vegas shooting last October, referring to scripture, dubbing the attack "an act of pure evil" and calling for national unity, in a nationally televised address just hours after the attack.

A day after the Las Vegas shooting, however, Trump called the shooter a "sick and demented man" and punted on questions over the whether the attack should prompt stricter gun control laws.

Cruz had recently been expelled from the school for disciplinary reasons and was enrolled elsewhere in the district, Broward County schools Superintendent Robert Runcie said. The gunman was believed to have been armed with a AR-15-style semiautomatic rifle and multiple magazines, according to Broward County Sheriff Scott Israel.

Peter Alexander contributed.