President Trump, in the wake of the deadliest school shooting since Sandy Hook, said Thursday there were so “many signs” the Florida suspect was “mentally disturbed” – suggesting more could have been done to report him to authorities.

“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!” Trump tweeted.

An error occurred while retrieving the Tweet. It might have been deleted.

The president plans to address the nation about the shooting at 11 a.m. ET on Thursday, the White House says. The flag was lowered to half-staff at the White House in honor of the victims.

Suspect Nikolas Cruz, 19, arrived at the Broward County Jail early Thursday morning after investigators questioned him for hours. The 19-year-old, who was a former student at Marjory Stoneman Douglas High School, was taken into custody “without incident” Wednesday by Coconut Creek police in a Coral Springs neighborhood, located just a few miles from the high school itself.

Seventeen people were killed Wednesday when the gunman opened fire at the Florida high school as panicked students fled the bloodbath. Cruz was charged Thursday with 17 counts of premeditated murder.

Cruz allegedly had an AR-15 rifle and “countless magazines.”

The school massacre immediately touched off another debate in Washington over whether gun control measures could have prevented the slaughter and should be considered anew in Congress.

Republican lawmakers are likely to push back on such calls, as they have after past shootings, focusing – as Trump did in Thursday’s tweet – on issues like mental illness.

The president tweeted on Wednesday that he had spoken with Florida Gov. Rick Scott and was working with law enforcement.

“My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school,” he tweeted.

Sen. Dianne Feinstein, D-Calif., a prominent gun control advocate, tweeted, “Another mass shooting. Reportedly another AR-15. My bill to ban assault weapons is ready for a vote. How long will we accept weapons of war being used to slaughter our children.”

Full details of Cruz’s mental condition are not publicly known. Broward County Mayor Beam Furr told CNN he was getting treatment at a mental health clinic for a while, but hadn't been there for more than a year.

Meanwhile, while Trump suggested the suspect should have been reported, police reportedly had been to his house before for various complaints.

Cruz had been expelled from the high school for “disciplinary reasons,” and math teacher Jim Gard told the Miami Herald that before Wednesday's fatal shooting, Cruz may have been identified as a potential threat.

The Associated Press contributed to this report.