The troubled 19-year-old man accused of the latest mass shooting in the United States is expected to make his first court appearance on Thursday.

Authorities have charged Nikolas Cruz with 17 counts of premeditated murder after he allegedly opened fire with an assault rifle Wednesday on students and teachers at the south Florida high school where he had been enrolled before being expelled last year for disciplinary reasons.

Police arrested Cruz outside Marjory Stoneman Douglas High School in Parkland, about 70 kilometers north of Miami.

They said Cruz, wearing a gas mask, began firing outside of the school and continued to shoot inside the building before eventually blending in with a group of students as they fled.

In addition to the 17 deaths authorities reported, the shooting left 15 others hospitalized.

Broward County Sheriff Scott Israel called the attack a “horrific, homicidal, detestable act.”

Israel told reporters the shooter was armed with multiple ammunition magazines and an AR-15 rifle, which authorities say he legally purchased a year ago after a background security check. Authorities offered no immediate explanation for the mayhem that unfolded at the end of the school day on a sunny afternoon.

“I’m absolutely sick to my stomach to see children who go to school armed with backpacks and pencils lose their lives,” Israel said. “This nation, we need to see something and say something. If we see different behavior, aberrant behavior, we need to report it to local authorities.”

As an investigation into the violence continued Thursday, new details emerged about the shooter from students and teachers who knew him.

Investigators looking into Cruz’s online activity, including his social media accounts, turned up what Israel described as “very disturbing” things. The county sheriff gave no details.

No official reason for his expulsion has been disclosed, although the Associated Press cited a student who said Cruz was kicked out of the school after a fight with his ex-girlfriend’s new boyfriend. He then enrolled at a different school.

Social media accounts that acquaintances of Cruz said were his showed him brandishing weapons.

Trump tweets reaction 

In a tweet early Thursday, President Donald Trump said 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!”

 U.S. Attorney General Jeff Sessions, in a Washington speech, said that gunmen carrying out mass shootings in the U.S. often “have given signals in advance” of their mental instability. “Perhaps we haven’t been effective in intervening,” he said. “We can and must do better.”

Trump issued a proclamation honoring the victims of the shooting and ordered American flags at U.S. installations around the world to be flown at half-staff through Monday. He planned a televised address on the tragedy for late Thursday morning.

After previous mass shootings in the U.S., some lawmakers have called for tightening the country’s weak gun control laws. But the Second Amendment to the U.S. Constitution enshrines gun ownership. Gun rights advocates in Congress, supported by the National Rifle Association, have defeated efforts to ban sales of certain types of guns or attachments to weapons that increase their firepower.[xyz-ihs snippet=”adsense-body-ad”]Wednesday’s shooting was one of 18 that have occurred at U.S. schools in the first 45 days of the new year.

The Miami Herald quoted a teacher at the school as saying Cruz threatened other students last year.

One student told reporters Cruz had a reputation in school of being “mentally unstable” and that he had threatened others.

“He was definitely not the kind of person who should have been allowed to have a gun,” the student said.

News accounts say Cruz’s mother died in November, leaving him in the care of a family friend where he was unhappy and subsequently went to live with another family that is cooperating with investigators.

Before going to a hospital to meet with victims and their families, Florida Governor Rick Scott gave his reaction to the attack, saying it was “absolutely pure evil.”

“The first you think about is, you know, God I hope this never happens to my family. Then, you think about, you’re furious,” Scott said. “How could this ever happen in this country? How could this happen in this state? This is a state that is focused on keeping all of our children safe.”

Florida mass shootings 

It is the second mass killing in Florida in nearly two years. In mid-2016, Omar Mateen, a 29-year-old security guard, killed 49 people and wounded 58 others in a terrorist and hate crime attack on a gay nightclub in Orlando.

Florida Attorney General Pam Bondi said the state will pay for both the funerals of the victims at the Parkland high school and counseling for their families. The local school system is also making grief counselors available for students.

Students told television networks that Wednesday unfolded like a normal school day until a fire alarm sounded. They said they then heard gunshots, sending people either running from the building or trying to hide.

Video taken inside a classroom showed students crouched under desks, screaming in shock and terror while gunshots rang out. Other students hid in closets, using their cellphones to text emergency messages to their parents.

Trump offered Florida authorities all the federal help they need and tweeted his “prayers and condolences” to the victims and their families. “No child, teacher or anyone else should ever feel unsafe in an American school,” he said.

Prior mass shootings have brought calls for tighter gun controls in the United States. Trump said after a November attack at a Texas church that stricter laws would not have stopped that gunman.

Senator blames Congress 

Democratic Senator Chris Murphy of Connecticut was among those Wednesday who urged more concrete action, blaming Congress for what he called a “scourge” of school shootings.

“It only happens here, not because of coincidence, not because of bad luck, but as a consequence of our inaction. We are responsible for a level of mass atrocity that happens in this country with zero parallel anywhere else,” Murphy said.

He added on Twitter, “If you’re a political leader doing nothing about this slaughter, you’re an accomplice.”

 Don’t tell me tomorrow isn’t the appropriate time to debate gun violence. If you’re a political leader doing nothing about this slaughter, you’re an accomplice.

Connecticut is home to Sandy Hook Elementary School in Newtown, where 20 elementary schoolchildren and six adults were massacred in a 2012 shooting.

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