WASHINGTON - President Donald Trump on Thursday appeared to voice support for raising the age at which someone could buy an assault rifle to 21, and he doubled down on his idea of arming some teachers as a deterrent for school shootings.

In a series of morning tweets, the president claimed the latter strategy would be far less costly than hiring guards and that "ATTACKS WOULD END!"

With his tweets, Trump echoed solutions he pushed during a "listening session" at the White House that included family of some of the 17 people killed by a gunman last week at Marjory Stoneman Douglas High School in South Florida.

"Highly trained, gun adept, teachers/coaches would solve the problem instantly, before police arrive," Trump said in one tweet.

The strategy of arming teachers has many critics, including some law enforcement officers and the National Education Association, the country's largest teachers lobby. In a statement Wednesday, NEA president Lily Eskelsen Garcia said, "Educators need to be focused on teaching our students."

In his tweets, Trump claimed his strategy had been mischaracterized by some news outlets and is more nuanced than reported. He said he envisioned only about 20 percent of teachers having concealed weapons and said they would have "military or special training experience."

"If a potential 'sicko shooter' knows that a school has a large number of very weapons talented teachers (and others) who will be instantly shooting, the sicko will NEVER attack that school," Trump said. Cowards won't go there ... problem solved. Must be offensive, defense alone won't work!"

In another tweet, Trump said school shootings typically last three minutes and it takes police and first responders five to eight minutes to arrive.

"Highly trained, gun adept, teachers/coaches would solve the problem instantly, before police arrive," Trump said. "GREAT DETERRENT!"

Some criminologists have questioned that reasoning, pointing out that some people who plan to commit mass shootings are prepared to die in the process.

Trump has offered no details on how such a program would work, how much it would cost and how school districts already strapped for cash would fund it. The Department of Education estimates there are 3.1 million public school teachers and 400,000 private-school teachers. Arming 20 percent of teachers would mean arming more than 700,000 people. (There were around 1.3 million active-duty U.S. military personnel in 2016.)

In an appearance Thursday morning on Fox News, White House counselor Kellyanne Conway said that none of the ideas floated during Thursday's meeting has been finalized and criticized reports focusing on arming teachers.

"To focus on that alone today is disingenuously covering the fuller discussion yesterday, and frankly it's disrespectful to the people who were in that room raising any number of different issues," Conway said.

Trump also spoke Wednesday of strengthening background checks and raising the age for purchasing weapons.

"I will be strongly pushing Comprehensive Background Checks with an emphasis on Mental Health. Raise age to 21 and end sale of Bump Stocks!" Trump said in a separate tweet. "Congress is in a mood to finally do something on this issue - I hope!"

Trump's advocacy for raising the age to 21 appeared to be in reference to regulating the purchase of an assault rifle - an active topic of debate in recent days.

But raising the legal age to buy an assault rifle from 18 to 21 is among the ideas opposed by the powerful National Rifle Association. In a statement this week, NRA spokeswoman Jennifer Baker noted that federal law prohibits anyone under the age of 21 from purchasing a handgun from a licensed firearm dealer.

"Legislative proposals that prevent law-abiding adults aged 18-20 years old from acquiring rifles and shotguns effectively prohibits them for purchasing any firearm, thus depriving them of their constitutional right to self-protection," Baker said.

In a series of tweets, Trump tweeted "I never said "give teachers guns' like was stated on Fake News @CNN & @NBC. What I said was to look at the possibility of giving 'concealed guns to gun adept teachers with military or special training experience - only the best. 20% of teachers, a lot, would now be able to

"....immediately fire back if a savage sicko came to a school with bad intentions. Highly trained teachers would also serve as a deterrent to the cowards that do this. Far more assets at much less cost than guards. A 'gun free' school is a magnet for bad people. ATTACKS WOULD END!

"....History shows that a school shooting lasts, on average, 3 minutes. It takes police & first responders approximately 5 to 8 minutes to get to site of crime. Highly trained, gun adept, teachers/coaches would solve the problem instantly, before police arrive. GREAT DETERRENT!

"....If a potential 'sicko shooter' knows that a school has a large number of very weapons talented teachers (and others) who will be instantly shooting, the sicko will NEVER attack that school. Cowards won't go there...problem solved. Must be offensive, defense alone won't work!

"I will be strongly pushing Comprehensive Background Checks with an emphasis on Mental Health. Raise age to 21 and end sale of Bump Stocks! Congress is in a mood to finally do something on this issue - I hope!"

As he continues a focus on school safety, Trump is scheduled to hold another listening session at the White House on Thursday with law enforcement officers.

Earlier this week, Trump directed Attorney General Jeff Sessions to propose regulations to ban "bump stocks" and other devices that turn semiautomatic firearms into "machine guns."

A bump stock was used by the shooter who opened fire on a country music festival in Las Vegas in October, killing dozens and immediately prompting calls for lawmakers or the administration to ban such devices through legislation or regulations, but efforts to pass a ban stalled in Congress.

The idea of arming teachers is not a new one for Trump, who often responds to mass shootings by proposing an increase in the number of law-abiding citizens who carry firearms and can stop a shooter.

In July 2015, following a shooting at a military recruitment center in Tennessee that left four Marines dead, Trump tweeted in all caps: "MILITARY LIVES MATTER! END GUN FREE ZONES! OUR SOLDIERS MUST BE ABLE TO PROTECT THEMSELVES! THIS HAS TO STOP!"

In November 2015, following a series of terrorist attacks in and near Paris that left 130 people dead, Trump criticized the city for having "the toughest gun laws in the world."

"Nobody had guns but the bad guys. Nobody had guns. Nobody," Trump said at the time at a campaign rally.

In December 2015, following a mass shooting in southern California that left 14 dead, Trump told reporters in Iowa that the victims of the shooting "could've protected themselves if they had guns."

"If you look at what happened in California, they didn't have guns, and they were slaughtered," Trump said.

In January 2016, Trump said at a campaign rally in Vermont that he wants to eliminate gun-free zones at schools.

"I will get rid of gun-free zones on schools, and - you have to - and on military bases," Trump said, to scattered cheers. "My first day, it gets signed, okay? My first day. There's no more gun-free zones."

In June 2016, following a mass shooting at a Florida nightclub, Trump said that "it's too bad some of the people killed over the weekend didn't have guns attached to their hips, where bullets could have thrown in the opposite direction." He said that "had people been able to fire back, it would have been a much different outcome."

"If some of those wonderful people had guns strapped right here - right to their waist or right to their ankle - and one of the people in that room happened to have it and goes 'boom, boom,' you know, that would have been a beautiful sight folks," Trump said in a radio interview at the time.

At the time, even leaders of the National Rifle Association said that arming drunk clubgoers was a bad idea. Trump then clarified his comment and said he "was obviously talking about additional guards or employees."

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