The expelled student accused of killing 17 people at his former South Florida high school is "sad, mournful, remorseful" and "he's just a broken human being," one of his attorneys told reporters Thursday.

After a judge ordered Nikolas Cruz, 19, held without bond as he faces 17 counts of premeditated murder, defense attorney Melissa McNeil said that Cruz was "fully aware of what is going on," but thad a troubled background and little personal support in his life before the attack.

Cruz appeared via video, in an orange jumpsuit and with his head slightly bowed, for an initial Broward County court hearing Thursday.

Meanwhile, investigators were scouring Cruz's social media posts for possible motives or warning signs of the attack. Several social media accounts bearing Cruz's name showed a young man fascinated by guns who had appeared to signal his intentions to attack a school long before the event.

Nine months ago, a YouTube user with the handle "nikolas cruz" left a comment on a Discovery UK documentary about the gunman in the 1966 University of Texas tower shooting. The user wrote beneath the YouTube video: "I am going to [do] what he did."

Other past comments by YouTube users with Cruz's name reportedly included one remark in September, saying: "Im going to be a professional school shooter." At a news briefing in Florida, Robert Lasky, the FBI special agent in charge, confirmed that the FBI had investigated that comment. But he said the agency couldn't identify the person in question.

In another post on Instagram, where he posted photos of himself in masks and with guns, Cruz wrote anti-Muslim slurs and apparently mocked the Islamic phrase "Allahu Akbar," or God is greatest.

Confusion also swirled after the leader of a white nationalist militia claimed that Cruz had trained with his armed group, a claim that drew wide attention but which could not be immediately verified.

The Republic of Florida militia's leader, Jordan Jereb, told researchers at the Anti-Defamation League that Cruz had been "brought up" into the group by one of its members, the nonprofit said in a blog post. ABC News also claimed to have spoken to three people who verified Cruz's membership, but some white nationalists expressed concern that the news outlet may have been targeted by a coordinated hoax.

The Republic of Florida calls itself "a white civil rights organization fighting for white identitarian politics" on its website, adding that its "current short-term goals are to occupy urban areas to recruit suburban young whites" in pursuit of "the ultimate creation of a white ethnostate."

A training video the group posted online shows members practicing military maneuvers in camouflage clothing and saluting each other, along with music that says: "They call me Nazi / and I'm proud of it."

In the weeks before the attack, on Gab, a social media network sometimes used by white nationalists, Jereb had recently praised Norwegian mass killer Anders Breivik as a "hero." He also posted a diagrammed strategy for using the Republic of Florida militia to create "lone wolf activists."

Jereb later told the Associated Press that he didn't know Cruz personally and that the group had no knowledge of his plans for the violent attack. "He acted on his own behalf of what he just did, and he's solely responsible for what he just did," Jereb said.

President Trump, in a televised address to the nation, decried the "terrible violence, hatred and evil" embodied by the attack.

"No parent should ever have to fear for their sons and daughters when they kiss them goodbye in the morning," he said, speaking from the White House. The president hailed first responders, offered to assist Florida officials "any way we can" and said he would visit Parkland soon. He urged social solidarity in the face of tragedy, but he did not call for any tightening of the country's gun laws.

Earlier, Trump tweeted a call for public vigilance.

"So many signs that the Florida shooter was mentally disturbed," the president wrote on Twitter. "Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

On Thursday, authorities combed the Marjory Stoneman Douglas High School in Parkland, a suburb of Fort Lauderdale, for more clues, struggling to piece together the chaotic and lethal series of events that unfolded the day before as the shooter stalked the halls and classrooms with an AR-15 semiautomatic weapon.