An orphaned 19-year-old who participated in paramilitary drills with a white nationalist group was charged with murder Thursday in the deaths of 17 people who were fatally shot at a huge Florida high school in the nation’s deadliest school attack in five years.

Nikolas Cruz legally purchased the AR-15 rifle used in the assault at Marjory Stoneman Douglas High School, law enforcement officials told The Associated Press.

As the criminal case against the suspect took shape, the leader of a white nationalist militia called the Republic of Florida said Cruz was a member of his group and participated in exercises in Tallahassee.

Jordan Jereb told The Associated Press that did not know Cruz personally and that “he acted on his own behalf” and is “solely responsible for what he just did.”

The group wants Florida to become its own white ethno-state. Jereb said his organization holds “spontaneous random demonstrations” and tries not to participate in the modern world.

He also said Cruz had “trouble with a girl” and that he believed the timing of the attack, on Valentine’s Day, was not a coincidence.

In a national address from the White House, President Donald Trump said he wanted America’s children to know, “You are never alone, and you never will be.”

He said no child should have to go to school in fear of getting killed. He planned to travel to Florida meet with victims’ families, explore how to better secure schools and to “tackle the difficult issue of mental health.”

At no point did Trump mention guns or how to control them.

Attorney General Jeff Sessions said he wants the Justice Department to study how mental illness affects criminal behavior, to better understand how law enforcement can use existing laws to prevent school shootings.

“It cannot be denied that something dangerous and unhealthy is happening in our country,” Sessions told a group of sheriffs in Washington. In “every one of these cases, we’ve had advance indications and perhaps we haven’t been effective enough in intervening.”

Republican Gov. Rick Scott said he’s already told Florida House Speaker Richard Corcoran that “if someone is mentally ill, he should not have access to a gun.”

Broward County Schools Superintendent Rob Runcie said “now is the time to have a real conversation about gun control legislation.” And if adults cannot manage that in their lifetimes, he said, students will do it.

Broward County Sheriff Scott Israel called for giving law enforcement more power to detain people who make threats.

“What I’m asking our lawmakers to do is go back to places like Tallahassee and Washington, D.C., to give police the power,” the sheriff said, to detain people who make graphic threats or post disturbing material online, and bring them involuntarily to mental health professionals to be examined.

Thirteen wounded survivors were hospitalized, including two people in critical condition.

Some bodies remained inside the high school Thursday as authorities investigate the crime scene, the sheriff said. The slain included a school athletic director and another adult who worked as a monitor at the school. Runcie called them heroes.

Cruz was ordered held without bond and booked into jail, still wearing a hospital gown from his treatment for labored breathing. The jail said he is 5-foot-7 and weighs 131 pounds.

It was the nation’s deadliest school shooting since a gunman attacked an elementary school in Newtown, Connecticut, in 2012. The overall death toll differs by how such shootings are defined, but Everytown For Gun Safety has tallied 291 school shootingsin America since 2013, and this attack makes 18 so far this year.

Trump lamented in a tweet that there were “So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior. Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!”

Cruz legally purchased the AR-15 used in the attack about a year ago, law enforcement officials told the AP. The officials, not authorized to discuss this publicly, spoke on condition of anonymity. Federal law allows people 18 and older to legally purchase long guns, including this kind of assault weapon.

FBI agent Rob Lasky said the agency investigated a 2017 YouTube comment posted with the screen name Nikolas Cruz that said “I’m going to be a professional school shooter.” Lasky said the FBI did a database review, but couldn’t determine the time or location of the post, or the true identity of the person making the comment.

Ben Bennight, whose YouTube username is “BenTheBondsman,” posted a video Wednesday saying he had spotted the comment on Sept. 24, took a screenshot, flagged it for YouTube and called an FBI office in Mississippi to report it. He said two FBI agents visited him the next day.

“I knew that I couldn’t just ignore that,” Bennight said. The FBI called him again Wednesday within about two hours of the shooting, and one agent interviewed him in person, he said.

“Basically they’re going to have to get with YouTube about where the comment originated, but I think they already know,” he said.

Authorities offered no immediate details about a possible motive, except to say that Cruz had been kicked out of the high school, which has about 3,000 students. Students who knew him described a volatile teenager whose strange behavior had caused others to end friendships with him.

Cruz’s mother, Lynda Cruz, died of pneumonia on Nov. 1 neighbors, friends and family members said, according to the Sun Sentinel . Cruz and her husband, who died of a heart attack several years ago, adopted Nikolas and his biological brother, Zachary, after the couple moved from Long Island in New York to Broward County.

The boys were left in the care of a family friend after their mother died, said family member Barbara Kumbatovich, of Long Island.

Unhappy there, Nikolas Cruz asked to move in with a friend’s family in northwest Broward County. That family agreed, and Cruz moved in around Thanksgiving. According to the family’s lawyer, who did not identify them, they knew that Cruz owned the AR-15 but made him keep it locked up in a cabinet. He did have the key, however.

Attorney Jim Lewis told the AP that the family is cooperating with authorities and had no idea he was planning the shooting.

He seemed like “just a mildly troubled kid who’d lost his mom” during the three months they lived together, Lewis said.

Lewis also said the family was not aware of any other weapons in the gun cabinet he had. Photos posted in an Instagram account linked to Cruz show half a dozen weapons displayed on a mattress and a box of ammunition.

Victoria Olvera, a 17-year-old junior, said Cruz was expelled last school year because he got into a fight with his ex-girlfriend’s new boyfriend. She said he had been abusive to the girl.

“I think everyone had in their minds if anybody was going to do it, it was going to be him,” said Dakota Mutchler, also 17.

Cruz carried multiple magazines of ammunition when he was taken into custody without a fight, about an hour after the shooting in a neighborhood about a mile away, authorities said.

(AP)