President Donald Trump on Thursday endorsed setting an age restriction of 21 on certain gun purchases, his most aggressive move yet in endorsing gun access changes after last week’s Florida high school massacre.

The president is eager to do something in the wake of last week’s gun massacre in Parkland, Florida, that left 17 students and teachers dead, aides say. Until late Wednesday afternoon, Trump had mostly focused on enhancing the background check system, improving information sharing among local, state and federal law enforcement, and addressing mental health problems that plague many mass shooters.

But during a “listening session” with Parkland survivors and those from other mass school shootings on Wednesday at the White House, Trump for the first time floated the notion of altering gun-purchasing laws.

“In addition to what we’re going to do about background checks, we’re going to go very strongly to age, that’s age of purchase,” Trump told the survivors.

[Shooting Survivors, Victims’ Families Tell Trump Emotional Stories]

Watch: Trump’s Clout on Gun Control is Limited, and House GOP Won’t Help

He doubled down on that the next morning and revealed just what age he will propose, most likely for the AR-15 assault rifle and similar weapons that have been used by many mass shooters.

“Raise age to 21 and end sale of Bump Stocks!” he tweeted, also alluding to his order that the Justice Department write regulations — despite bipartisan questions about whether it has the legal authority to do so — to ban devices that make semi-automatic weapons fire like automatic ones.

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!

— Donald J. Trump (@realDonaldTrump) February 22, 2018

Sen. Marco Rubio, R-Florida, also endorsed a minimum age for the purchase of assault-style firearms Wednesday night during a town hall in Broward County, where Parkland is located.

Fred Guttenberg, whose 14-year-old daughter was killed in the high school rampage, asked Rubio if guns were “the factor in the hunting of our kids?”

“Of course they were,” Rubio told him. “Number one, Fred, I absolutely believe that in this country if you are 18 years of age you should not be able to buy a rifle and I will support a law that takes that right away.”

Trump’s endorsement of altering laws to prohibit individuals under 21 from buying certain guns comes less than 24 hours after Parkland father Cary Gruber, whose son Justin survived the shooting and told Trump his story, pleaded with the president to help keep guns out of the hands of troubled teens.

Watch: Students March for Gun Control

“It’s not left and right, it’s not political. It’s a human issue. People are dying. And we have to stop this. We have to stop,” Gruber said during the White House session. “If he’s not old enough to buy a drink — to go and buy a beer — he should not be able to buy a gun at 18 years old. I mean, that’s just a common sense. We have to do common sense. Please, Mr. Trump, these are things we have to do.”

The president on Thursday morning also expressed his sense lawmakers might act as students, teachers and parents across the country demand changes aimed at preventing additional school massacres.

“Congress is in a mood to finally do something on this issue — I hope!” he wrote.

Trump also on Wednesday floated the notion of providing guns and “special training” to about “20 percent” of teachers and staff members. On Thursday morning, he suggested a “large number” of teachers might have to be armed and trained.

“It only works where you have people very adept at using firearms, of which you have many, and it would be teachers and coaches,” Trump tweeted. “They’d go for special training. And they would be there, and you would no longer have a gun-free zone. A gun-free zone to a maniac — because they’re all cowards — a gun-free zone is, ‘Let’s go in and let’s attack, because bullets aren’t coming back at us.’”

The president also covered some other bases — expressing confidence that  National Rifle Association Executive Vice President Wayne LaPierre and and Chris Cox, head of the NRA’s lobbying arm, will “do the right thing.”

What many people don’t understand, or don’t want to understand, is that Wayne, Chris and the folks who work so hard at the @NRA are Great People and Great American Patriots. They love our Country and will do the right thing. MAKE AMERICA GREAT AGAIN!

— Donald J. Trump (@realDonaldTrump) February 22, 2018