The family of the 21-year-old man identified as the gunman who fired into a crowd celebrating the Fourth of July in Highland Park, Ill., appeared to have deep roots in the community. His father ran for mayor; his grandfather was born in the city and was buried in a cemetery 13 miles outside of town.

But there were signs of trouble in the family.

In April 2019, someone who knew Robert E. Crimo III, the man identified as the gunman, called the police to say that the teen had attempted suicide, the police said.

Four months later, a family member contacted the authorities, reporting that Mr. Crimo had threatened to “kill everyone.” Police officers removed 16 knives, a dagger and a sword from the home, but there was no probable cause to arrest him at that time, Deputy Chief Christopher Covelli of the Lake County Sheriff’s Office told reporters on Tuesday.

As a portrait of Mr. Crimo emerged, the authorities said they were looking at videos he had posted on social media, some of which showed disturbing drawings of mass shootings.

“We’ll look at them and we’ll see what they reveal,” Chief Covelli said.

Mr. Crimo’s grandfather, Robert Crimo, who died in 2018, was born in the city in 1929, according to his obituary. When his son, a deli shop owner, ran for mayor against the current mayor, Nancy Rotering, he said he wanted to improve local ordinances to help downtown businesses thrive.

“Highland Park is my home and always will be,” Robert Crimo Jr. wrote in an election questionnaire published in a local publication.

Ms. Rotering, who has been mayor of the city for 12 years, described the 2019 contest as a “fine” race devoid of nasty campaigning. She won re-election with more than 73 percent of the vote, according to the Lake County clerk’s office.

The Crimos’ longtime connection to the city is typical of many residents in Highland Park, a community comprised of many intergenerational families. Ms. Rotering said that she knew Robert Crimo III when he was about 6 years old and a Cub Scout in a troop she led.

There was nothing then out of the ordinary about Mr. Crimo, who as a child learned to build fires and camp in the woods like the other Cub Scouts, she said.

“He was just a little boy,” Ms. Rotering said.

He did not go to college, but spent time on social media as an aspiring artist and rapper on YouTube, according to his uncle, Paul Crimo, who spoke with a local TV station, FOX 32.

Mr. Crimo’s music videos seem to reference mass shootings. One video includes cartoon images of a gunman pointing a large rifle, and of other figures spurting blood. Later in the video, the gunman lies in a pool of blood near police cars.

Another video shows Mr. Crimo with a newspaper displayed on the wall behind him carrying a headline about the killing of Lee Harvey Oswald, who assassinated President Kennedy in 1963. Mr. Crimo, who identified himself as “Awake the Rapper,” sits on a bed in front of the newspaper. The word “Awake” was tattooed over his left eyebrow.

Investigators are reviewing videos Mr. Crimo made and they “are going to be part of the investigation,” Chief Covelli said.

Paul Crimo said that he shared a family house with the younger Mr. Crimo and had spoken to him on Sunday evening. “I saw no signs of trouble. And if I did see signs, I would have said something,” he said in the interview with FOX 32. “I’m deeply heartbroken and I’ll be heartbroken for the rest of my life.”

The authorities said Mr. Crimo had acquired five firearms after the knives were seized from his home, including two AR-style rifles, some pistols and possibly a shotgun. But the uncle said he did not know where Mr. Crimo might have acquired the gun used in the shooting, and was not aware of whether Mr. Crimo had any mental health issues.

Mr. Crimo was a “real quiet kid,” the uncle said. “He keeps to himself and he doesn’t express himself out. He just, like, sits down on his computer. There’s no interaction between me and him.”

Jeremy Cahnmann, who directed an after-school sports program at Lincoln Elementary School about a decade ago, said what stood out to him was that Mr. Crimo and his brother were often left waiting at the end of the day.

“When the program ended at 4:30, everyone else had their parents or their grandparents or their caregivers pick them up and take them home. And the last kids waiting there every day were the Crimo kids,” he said.

He said teachers at the school talked about how Mr. Crimo’s parents were difficult to reach. “It was a common occurrence,” he said. “If they needed to get ahold of somebody in that house, they just couldn’t.”

Mr. Crimo, who was about 10 and went by “Bobby” at the time, was “average,” he said. “He was quiet, he wasn’t disruptive, and he wasn’t necessarily a problem any more so than another kid at 10 years old is.”

Nicolas and Andres Lopez, brothers who later went to Highland Park High School with Mr. Crimo, said they used to be friends with him.

“There was a group of five of us, we used to skateboard in Highland Park and Highwood,” said Nicolas Lopez. “We’d smoke and do high school stuff.”

Mr. Crimo at one point dropped out of high school, but the brothers said there was nothing during the time when they were friends to suggest a problem.

“He was always quiet and reserved but nice,” Andres Lopez, 23, said. “He wasn’t a quiet kid who was dark then. He was quiet because he was nerdy. He wasn’t sinister.”

In 2017, the Lopezes’ older brother, Anthony LaPorte, died of a heroin overdose.

Mr. Crimo spoke at the funeral, the brothers recalled.

“He was very upset, saying my brother was one of his only friends,” Nicolas Lopez said.

He said that he believed a woman Mr. Crimo was dating also broke up with him around that same time.

“That’s when he started acting weird,” Andres Lopez said. “He was reclusive.”

Alfredo Balbuena, 22, said he knew Mr. Crimo from Highland Park High School and described him as “a quiet, lonely kid” who often dressed in black.

“He kept to himself,” Mr. Balbuena said. “He wore black band stuff, emo-ish stuff, and had a lot of tattoos.”

Michael Levenson contributed reporting.