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Investigators are combing through evidence in the wake of New York City’s deadliest mass shooting since 2000, trying to figure out more about the gunman who opened fire Monday in Midtown, killing four people, including a police officer, and injuring one.

The shooting at 345 Park Avenue, a glossy skyscraper near 52nd street and blocks from busy tourist sites like Rockefeller Center and the Museum of Modern Art, happened as office workers across the city were filing out of their workplaces during the busy evening commute. It is among at least 254 mass shootings in the US so far this year, according to the Gun Violence Archive.

The building is home to large global companies, including investment firm Blackstone and the National Football League – which the gunman, a competitive football player in his youth, had grievances with, according to a law enforcement source.

Here’s what we know:

The gunman

The gunman, 27-year-old Shane Devon Tamura from Las Vegas, drove across the country in the days preceding the attack and arrived in New York City Monday afternoon, police said.

Police said they are investigating Tamura’s motives, and the FBI said its initial search of internal systems had not turned up information about gunman. Tamura had a “documented mental health history,” NYPD Commissioner Jessica Tisch said during a news conference Monday night.

Tamura had a suicide note in his pocket alleging he suffered from CTE, or chronic traumatic encephalopathy, a brain disease linked to head trauma, according to a source with knowledge of the investigation. Tamura had once been a competitive football player, multiple sources told CNN.

In the note, the source said, he asked that his brain be studied and wrote: “You can’t go against the NFL, they’ll squash you.” The short note was scribbled over three pages and found by investigators after the shooting, the source said.

CTE is commonly associated with football players, and studies show that repetitive hits to the head can result in the disease.

The NFL’s offices are located on the fifth floor of the Park Avenue building.

Former friends and classmates have expressed shock at the news, saying they remembered Tamura as unproblematic.

“When I knew him, he was a great teammate. He was a great guy in general. He didn’t cause any problems, actually at all, in the locker room or on the field. He was just a guy that really enjoyed the sport, not problematic at all,” said a former high school friend, according to CNN affiliate KABC.

Another childhood friend, who asked not to be identified, remembered Tamura as being a nice guy and a good athlete.

Both friends had not spoken to Tamura in many years.

How the attack unfolded

Shortly before 6:30 p.m., surveillance cameras captured Tamura leaving his car near the Park Avenue building and entering with an M4 assault-style rifle in hand, Tisch said. Upon entering the lobby, he began “spraying it with gunfire,” Tisch added, hitting a police officer, a security guard, and another man and woman.

Witnesses in the area described hearing windows shattering and a loud noise, according to the Associated Press.

“It felt like it was a quick two shots and then it was rapid fire,” Nekeisha Lewis, who was getting dinner with friends nearby, told the AP. She saw a man run from the building saying, “Help, help. I’m shot.”

After opening fire in the lobby, the gunman went to the elevator, where a woman ran out – who he allowed to pass unharmed, Tisch said.

He then took the elevator to the 33rd floor, where the woman had been heading, according to a law enforcement official. It’s the floor where the building’s owner, real estate company Rudin Management, has its offices. Tamura then opened fire again, killing one person before shooting himself in the chest and dying of the self-inflicted wound, Tisch said.

The victims

Only one of the five people shot survived, and was in critical but stable condition at the hospital, police said on Monday night.

One man and one woman who were shot were brought to Bellevue Hospital and both died, police said. Another woman was found dead on the 33rd floor.

Their names are being withheld until their families can be notified, said the police commissioner.

The fourth fatality was NYPD officer Didarul Islam, a 36-year-old immigrant from Bangladesh who had worked in the police force for three and a half years. He had two young sons, and his wife is pregnant with their third child, police said.

Islam had been off-duty at the time, but was working security in the building when Tamura stormed the lobby and shot him. Officials, including Mayor Eric Adams and Gov. Kathy Hochul, expressed their condolences to his family and called him a hero.

Adams met with Islam’s family on Monday, saying afterward that Islam had been his father’s only son.

“Everyone we spoke with stated he was a person of faith and a person that believed in God and believed in living out the life of a godly person,” Adams said.

Just past midnight, officials lined the streets outside a hospital to perform a “guard of honor” as Islam’s body was transferred to an ambulance, with some saluting and some holding their hands over their hearts as he was wheeled out.