(CNN) -- After each mass shooting, those who knew the shooter start asking questions. What could we have done to stop this? Where should we have stepped in? Did we miss the signs?

In the case of 19-year-old Nikolas Cruz, numerous missed red flags have left a community to grieve and a nation to wonder if his February 14 massacre at a South Florida high school could have been stopped.

Neighbors were raising concerns about Cruz as early as age 9, when he got in a rock-throwing fight with another boy. As he became a teenager, he showed a propensity for violence toward small animals, expressed enthusiasm about guns and knives and even began introducing himself as "a school shooter."

Deputies with the Broward County Sheriff's Office were alerted to Cruz's behavior many times over the years -- they have released details of numerous calls to the Cruz home -- and two deputies are now under internal investigation for their handling of his case. Even the FBI received two tips about Cruz and the potential threat he posed to schools, but the bureau never chased the leads far enough.

Cruz faces 17 charges of premeditated murder for the massacre and has yet to enter a plea.

While the individual incidents may not have foreshadowed a mass shooting, his pattern of disturbing behavior grew over the years, begging the question: Why did no one intervene?

Emma Gonzalez, one of Cruz's classmates at Marjory Stoneman Douglas High School, said in a speech last week that his peers reported his behavior "time and time again."

"Since he was in middle school," she said, "it was no surprise to anyone who knew him to hear that he was the shooter."

'Evil kid'

Nikolas was adopted at an early age by Roger and Lynda Cruz. Roger died of a heart attack in 2004, leaving Lynda to raise Cruz and his adopted younger brother as a single mother in Broward County, Florida.

His neighbors describe Cruz as a troubled kid whose dark behavior clashed with the sunny image of South Florida suburbia with its homes in neat rows, framed by palm trees.

"He had a very cold stare," said Rhonda Roxburgh , a neighbor of the young Cruz. Over the years, he got in several fights with other kids, and once bit a neighbor's ear, injuring him. Cruz would often carry a pellet gun around the neighborhood, shooting squirrels, Roxburgh told CNN.

Many of Cruz's earliest encounters with police were prompted by Lynda, an exasperated mother whose adoptive son was often unruly, disrespectful and sometimes violent.

"Police were there almost every other week," Speno said.

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Sheriff's office records show deputies responded to the Cruz home on several occasions after the youth got into fights with his mother or his brother. In one instance in November 2012, police responded after Cruz, then 14, hit his mother with a plastic hose from a vacuum cleaner, according to the details of calls to the Cruz residence released by the Broward County Sheriff's Office on Friday.

Just a few months later, Cruz called Lynda "a useless bitch" and threw a chair, a dog bowl and a drinking glass across the room after she took away his Xbox. Police responded, but a counselor from the nearby Henderson Behavioral Health facility felt it unnecessary to invoke the Baker Act, a Florida law which allows police to take a mentally ill person into custody.

Another allegation of animal abuse came in November 2014, when a neighbor told police a red-haired boy was possibly shooting at chickens with a BB gun. Deputies discovered the suspect was Cruz, 16, who had an airsoft rifle. The teen admitted to firing the gun but denied taking shots at any chickens.

Lynda locked the rifle away, and the owner of the chickens chose not to press charges.

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'I knew it was him'

Cruz was just as unruly at school as he was at home, racking up more than two dozen infractions between 2012, when he was in middle school, through January 19, 2017, shortly before he withdrew from Stoneman Douglas, according to Cruz's disciplinary records from Broward County Schools.

He frequently faced disciplinary action for unruly behavior, ranging from the use of profanities to fighting other students. Parent conferences, detentions and suspensions were regular occurrences.

Cruz's behavior made the students and teachers around him uncomfortable, according to Laurel Holland, who taught Cruz's English class in 2016. He was disruptive and unruly, cursing her out during a midterm exam and being caught with a gun-related object in his backpack.

Cruz in an undated family photo, probably from late 2017.

When she first heard about the shooting at Stoneman Douglas, Holland "knew it was him."

"He fell through the cracks because we don't know what to do," said Holland, who is now retired and lives in North Carolina.

As early as February 2016 Cruz had started to make alleged threats about attacking a school. That month the county sheriff's office received "third hand information" from a neighbor's son who claimed Cruz had talked about shooting up a school on his Instagram, where he posted pictures of himself with guns. He was 17.

A deputy ultimately determined Cruz had knives and a BB gun, and he forwarded the information to Stoneman Douglas' school resource officer. The deputy's handling of the incident is now the focus of an internal affairs investigation at the sheriff's office.

'A vulnerable adult'

More concerns about Cruz's mental health were raised in September 2016, days after his 18th birthday. After a breakup with a girlfriend, Cruz took to Snapchat to post images of himself cutting both his arms, according to a report from the Florida Department of Children and Families. He also began expressing interest in buying a gun.

The report from the Florida Department of Children and Families said his struggles with depression and ADHD impaired "his ability to cope with the demands of everyday life without the use of medication."

Lynda told investigators from the DCF that her son had written racial slurs and drawn "a Nazi symbol" on his bookbag.

A DCF report — which described Cruz as a "vulnerable adult due to mental illness" — concluded that his "final level of risk is low" because he lived with his mother, was receiving mental health treatment and was attending school.

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'A professional school shooter'

After he was involved in an assault in January 2017, Stoneman Douglas asked for a threat assessment to determine whether the young man was a danger to his peers . It's unclear whether the assessment was ever done, but Cruz soon transferred to an alternative school nearby for students with specific learning needs.

Last September, the FBI received a tip from Ben Bennight , a Mississippi man who posts video blogs about being a bail bondsman to YouTube. A comment had been left on one of his videos, he said, under the username "nikolas cruz."

"Im going to be a professional school shooter," it said.

Two agents from the bureau's field office in Mississippi interviewed Bennight in person the next morning, but he told them he knew nothing else about the comment or the user who posted it.

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The information was never shared with local authorities and there was no follow up. On February 14, after Cruz had been arrested for the shooting, an FBI agent in the Miami field office called Bennight, and agents from the Mississippi office visited soon after.

"When the FBI said it was the same name," Bennight told CNN, "the first thing that went through my mind was, 'Wow, I hope you were at least watching this guy that I alerted you to months ago.'"

The FBI would later reveal it closed its inquiry into the comment because it could not positively identify the individual behind the post. On Friday, the Senate Judiciary Committee was briefed by the FBI and Google, YouTube's parent company, which confirmed that the comment was, in fact, posted by Nikolas Cruz.

'I think I am going to kill people'

But Cruz didn't just make disturbing comments on YouTube videos. Like many teenagers, he was a frequent user of Instagram, where his accounts were rife with posts about weapons. On January 6, 2016, he posted a picture of a shotgun, looking for advice from his followers, according to the Instagram account reviewed by CNN.

"I plan on getting this but I need more information on it so if someone could give advice on how much I'm spending and background cheeks (sic) please to god let me know," he wrote in the caption, followed by a comment: "I plan on putting a scope on it for accuracy."

In other posts Cruz is seen holding a knife or what appears to be a BB gun in front of his face, which is sometimes obscured by bandanas, masks and hats. He also took selfies in the mirror, but they were not typical. In them, he's wearing camouflage, body armor and tactical vests.

One of his posts showed a disemboweled frog, surrounded by blood. Cruz killed it, he said, because one had killed his dog.

His mother's death

Last fall, Lynda contracted the flu. She passed away in November after it mutated into a bout of pneumonia

Nikolas Cruz at Lynda Cruz's funeral, with an urn of his mother's ashes.

Her funeral was a small affair, attended only by Cruz, his brother, Rocxanne Deschamps — a former neighbor who would take the boys into her care — and her boyfriend, Paul Gold, who described the funeral to CNN.

"The boy was stoic. Not a tear. Not an emotion," Gold said, recalling Cruz's behavior at the service. "I asked him if he was upset. He said: 'I'm upset because nobody came, and nobody cares about my mother.'"

It was a pivotal time for Cruz. His attorneys say his mother's death exacerbated his struggle with his depression and mental illness.

'I'm dealing with a bunch of things right now'

On November 24, the Palm Beach Sheriff's Office received a call from Deschamps' son Rock, who told police Cruz might have hidden a gun in their back yard. Weapons weren't allowed in the home, he said.

It's unclear whether deputies conducted a search of the property, but records from the sheriff's office show deputies didn't find proof to support Rock's claim.

Days later, the Deschamps called police again. It was Rocxanne, saying that Rock and Cruz had gotten into a fight. Cruz left, telling his host family that "he was going to get his gun and come back."

"He has put the gun to others' heads in the past," and had "bought tons of ammo," she told police.

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The caller told the operator that he got mad and started punching walls when Rock "came at me and threw me on the ground," before he was kicked out of the house.

"The thing is, I lost my mother a couple days ago, so, like, I'm dealing with a bunch of things right now," he said.

A deputy found Cruz in a local park. He said he'd misplaced a picture of his late mother and got upset prior to his fight with Rock. When he was taken back to the Deschamps' home, Cruz and Rock "hugged" and Cruz apologized.

The Deschamps, who said Cruz had been struggling since the death of his mother, declined to press charges.

After a few weeks, Cruz left the Deschamps' home and moved in with James and Kimberly Snead, who were aware Cruz was depressed following his mother's death. They also knew about his weapons and made him get a gun safe before he moved in last November.

Nikolas Cruz appears in Broward County Circuit Court on February 19.

"To me, the depression was more stemmed from loss -- losing his mother, not from all the things they said about him being bullied, or by things that happened in school," said Kimberly Snead. With the gun safe, she said, "everything'd be locked up. It really didn't concern me."

'A school shooter in the making'

Yet before November was over, another caller was warning the sheriff's office about Cruz, who by this time was collecting guns and knives.

"Concerned he will kill himself one day," police records say, recounting the caller's concerns, "and believes he could be a school shooter in the making."

But Cruz no longer lived in Parkland, according to the caller, who was from Massachusetts. At the time, he was staying in Lake Worth, so the Broward County deputy referred the caller to the Palm Beach Sheriff's Office, which would have jurisdiction.

The deputy's response to the call is also the subject of an internal affairs investigation by the Broward County Sheriff's Office. But it wasn't the last time someone warned authorities about Cruz, his weapons and the threat he posed.

'Something's going to happen'

A woman close to the 19-year-old called an FBI tip line and painted a picture of a troubled young man who she believed was "going to explode."

"I just want to, you know, get it off my chest," she said, "in case something does happen and I do believe something's going to happen."

Cruz had an arsenal of guns and knives, she said, and used the latter to mutilate small animals like frogs and birds. She recounted one occasion when a bird had flown into a sliding glass door at the Cruz home and fell to the ground.

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"He brought the bird into the house," she said. "He threw it on his mother's kitchen counter and he started cutting it up."

"That to me would be a red flag," the tipster said, according to a transcript of the call reviewed by CNN.

She went on to provide the FBI with user names for three Instagram accounts belonging to Cruz, where he posted pictures of the animals. In another post he wrote, "I want to kill people."

Last week, the FBI confirmed it failed to act on the January 5 tip. The bureau's protocols weren't followed, it said , and the information was never shared with agents from the Miami field office.

'Fully aware of the threat?'

A little more than a month later, on Valentine's Day, Cruz walked into Building 12 on the Stoneman Douglas campus, pulled the fire alarm and began unleashing a torrent of gunfire.

Broward County Deputy Scot Peterson, the school's armed resource officer, rushed to the western entrance of the building but balked at going inside, according to Broward Sheriff Scott Israel.

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With the cacophony of gunshots and the fire alarm blaring, Peterson chose not to go after the shooter. "He never went in," Israel said. That revelation led Peterson to resign last week amid criticism that he should have done more to stop Cruz.

Israel has been critical of Peterson but told CNN Sunday he couldn't comment more broadly on his department's failure to recognize the threat posed by Cruz because his office is under investigation. "I can only take responsibility for what I knew about," he said.

In a letter sent to Florida Gov. Rick Scott on Saturday, state Rep. Bill Hager called for Israel to be removed from his post for his deputies' "unfathomable inaction."

"The Sheriff was fully aware of the threat this individual present to his community and chose to ignore it," Hager wrote. "Sadly, he was not the only one that ignored it. DCF, Broward County Schools, the FBI and the BSO all had the pieces to put this puzzle together, but failed to communicate."