(CNN) Police say an Uber car dropped off Nikolas Cruz at his former school around 2:19 p.m. on Wednesday.

Within 10 minutes, authorities say he gunned down 17 people at Marjory Stoneman Douglas High School and left campus undetected in a crowd of students.

Now, as the 19-year-old gunman begins his journey through the criminal justice system, a community is in mourning and investigators are looking for answers.

A vigil was held Thursday for the victims of a school shooting in Parkland, Florida.

Latest developments

• Broward Sheriff's deputies were called to the Cruz family home 39 times since 2010.

• The shooter's former neighbor said Cruz pointed a BB gun at homes and did target practice in the neighborhood.

• Cruz had an Instagram account with posts that include photos of a rifle and a collection of firearms on a bed.

• The shooter legally purchased the firearm used in the shooting at a gun store in Coral Springs, Florida.

What we know about the shooter

The Sheriff's Office identified the 17 victims Thursday afternoon. As he read the names, the Sheriff Scott Israel acknowledged his "special friend," the school's assistant football coach Aaron Feis , who threw himself in front of students as bullets flew.

Cruz confessed to police to being the gunman, according to a probable cause affidavit. His public defender described him as a "deeply disturbed, emotionally broken" young man who is coming to grips with the pain he has caused.

"He's gone through a lot in a very short period of time and that does not minimize the loss of those families, but we have to put that into the proper light," Gordon Weekes said. "He is suffering from significant mental illness and significant trauma and he has some very difficult decisions to make shortly and we're going to assist him with those decisions."

Meanwhile, Cruz's digital footprint offers disturbing glimpses into his mind.

He hurled slurs at blacks and Muslims, and according to the Anti-Defamation League, had ties to white supremacists. He said he would shoot people with his AR-15 and singled out police and anti-fascist protesters as deserving of his vengeance.

Posts under videos on YouTube and other sites by someone using the name Nikolas Cruz include threatening comments, such as:

"I whana shoot people with my AR-15."

"I wanna die Fighting killing s**t ton of people."

"I am going to kill law enforcement one day they go after the good people."

On an Instagram account under the name @Nikolascruzmakarov, his profile picture shows him with a mask around his face, wearing a Make America Great Again hat. Other posts include a photo of a rifle, a collection of firearms on a bed, and a photo taken through a scope looking out a window.

A video blogger said he warned the FBI in September about a possible school shooting threat from a YouTube user with the same name as Cruz. An FBI agent confirmed that a field officer in Jackson, Mississippi, received the tip and interviewed the person who shared it. But no additional information was found to help identify the person who posted the comment and no connection was made to south Florida, said Robert Lasky, FBI special agent in charge of the Miami division.

A former neighbor said Cruz pointed a BB gun at homes and did target practice in the neighborhood. In video the former neighbor shared with CNN, Cruz is on a back patio, wearing only boxers and a red baseball cap, brandishing a pistol that appears to be a type of BB gun called an airsoft pistol.

The gun debate returns

Cruz is being held without bond after a brief hearing Thursday in Broward County court. Appearing by video from jail, he said nothing except to confirm his name. He nodded when he was told he couldn't post bail.

US President Donald Trump said he is making plans to visit Parkland to meet with families and local officials. He pledged to meet "the nation's governors and attorney generals where making our schools and our children safer will be our top priority."

Cruz purchased the firearm used in the shooting, an AR-15 style rifle, legally in the state of Florida nearly a year ago, according Peter J. Forcelli, special agent in charge of the Miami field division of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

What we know about the shooting

After the Uber driver dropped him off, Cruz entered the school through the east stairwell around 2:21 p.m. He was carrying a rifle inside a soft black case, according to a law enforcement timeline. At some point Cruz activated a fire alarm, sending people outside, unaware of what was to come. Some students said the alarm caused confusion because there had been a fire drill earlier that day.

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Cruz roamed the halls, targeting those huddled in classrooms on the first floor before going to the second floor and shooting a victim in another room.

Some students texted goodbyes to loved ones. Others posted social media images of chairs overturned in classrooms and floors stained with blood.

In one cellphone video, a student cried, "Oh my God! Oh my God!" as gunshots popped in rapid succession in the background.

Survivors described hearing anguished cries of the wounded. Freshman Kelsey Friend said she heard her teacher being shot dead in the doorway of their classroom while she and classmates hid near the teacher's desk.

Moments earlier, they had left the room for the fire alarm, but returned after hearing gunshots. The teacher unlocked the door, allowing students to get back in, she said. She ran in, thinking her teacher was behind her. Then, she heard the sound of gunshots and the shooter walking down the hallway.

Until that moment, she'd hoped it was an active-shooter drill, with police officers firing blanks, "until I saw my teacher dead on the floor."

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Cruz dropped his rifle and backpack on the third floor before running out of the building and blending in with evacuating crowds.

He left the scene and bought a drink at a Subway sandwich store, then sat at a McDonald's for a few minutes, the timeline states.

Investigators identified Cruz from school security videos. About 40 minutes later, a Coral Springs officer detained him as he was walking along the side of a road.

'Nobody saw this kind of aggression'

Cruz had been expelled from the high school over disciplinary problems, Broward County Public Schools Superintendent Robert Runcie said, without providing specifics. The school is closed for the rest of the week, as the district offers grief counseling to students and their families.

Cruz was adopted. His adoptive mother, Lynda Cruz, died in November of pneumonia. Kathie Blaine, a cousin of Cruz's adoptive mother, said his adoptive father passed away years ago.

Broward Sheriff's deputies were called to the Cruz family home 39 times since 2010, according to documents obtained by CNN. The sheriff's office received a range of emergency calls that included: "mentally ill person," "child/elderly abuse," "domestic disturbance," "missing person." Details of those calls are not immediately available; most of them are marked "no written report," so it's impossible to know if they involved Nikolas Cruz.

After Lynda Cruz's death, the family of someone Cruz met at the high school let him stay in their home, said Jim Lewis, attorney for the host family.

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That family knew he had a gun, Lewis said. "They had it locked up, and believed that that was going to be sufficient, that there wasn't going to be a problem."

Asked if the family had seen troubling signs, Lewis said they "saw some depression" over his adoptive mother's death.

"Obviously, he'd lost his mom. But they helped him get a job at a Dollar Tree store. They got him going to an adult education so he could try to get his GED and he seemed to be doing better," Lewis said.

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The family was unaware of any mental illness beyond depression, Lewis said.

"They didn't see that. They didn't see a mentally ill person, or they wouldn't have let him live under their (roof)," Lewis said.

"Nobody saw this kind of aggression or motive in this kid, that he would do anything like this," he said.