Wearing body armor and wielding an assault-style rifle, the alleged gunman, identified by police as 27-year-old Shane Tamura, burst through the lobby doors of a 44-story office tower in the heart of Midtown Manhattan on Monday, shooting victims at random, according to authorities.
The attack at the 345 Park Ave. high-rise, just two blocks from St. Patrick's Cathedral and Rockefeller Center, occurred around 6:30 p.m. local time and left four victims, including an off-duty police officer working security at the building, dead and one wounded as the shooter made it all the way to the 33rd floor, where police say he died by suicide.
A motive for the Midtown mass shooting remains under investigation.
Investigators are analyzing a note found in Tamura's pocket in which the former high school football player claimed he suffered from chronic traumatic encephalopathy (CTE) and asked that his brain be studied, police sources told ABC News.
CTE is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes, including football players, hockey players and boxers.
MORE: What we know about Shane Devon Tamura, the suspect of the Midtown Manhattan shootingNew York City Mayor Eric Adams said on Tuesday that based on the suspect's note, investigators allege he was focused on the headquarters of the National Football League (NFL), which is housed in the building.
New York Police Commissioner Jessica Tisch said Tamura had a "documented mental health history," but did not elaborate.
MORE: Manhattan shooting victims: What we know about those killedHere is a timeline of the suspect's movements before and during the attack:
July 26 -- Driving a black BMW, Tamura, once a licensed private investigator in Nevada, is believed to have left his home in a gated community in Las Vegas, Nevada, according to police. Investigators say his vehicle traveled through Colorado later that day.
July 27 -- Tamura's car, according to investigators, is tracked driving through Nebraska and Iowa.
July 28 -- The suspect's car is tracked to Columbia, New Jersey, in Warren County, about 70 miles from New York City. Tamura's car was in Columbia at 4:24 p.m., and shortly thereafter entered New York City, according to police.
July 28 -- At 6:28 p.m., the suspect double-parks his BMW in front of the building at 345 Park Ave., according to police. He is captured on a security video walking up to the building's glass revolving door, wearing sunglasses, a dark jacket and shirt over his body armor and carrying at his right side what police describe as a high-powered M4 rifle, authorities said.
The suspect, who obtained a concealed carry license in Nevada in 2022, enters the lobby of the building "spraying it with gunfire" as he moves toward the elevators, according to police.
The gunman fatally shoots an off-duty NYPD officer, Didarul Islam, who is working security at the building and confronts the shooter in an effort to protect others, officials said.
Aland Etienne, another security guard at the building, is also fatally shot confronting the suspect, according to Manny Pastreich, president of the 32BJ branch of the Service Employees International Union, of which Etienne was a member.
The suspect allegedly kills one other person before taking an elevator to the 33rd-floor offices of real estate management firm Rudin Management, where he fatally shoots a fourth victim before fatally shooting himself in the chest with the M4 rifle, according to police.
July 28 -- In the immediate aftermath of the rampage, police find a handwritten note in the suspect's pocket in which he says he has CTE and accuses the NFL of concealing the dangers to players' brains to maximize the league's profits, police sources tell ABC News.
In his note, the suspect blames playing football for his CTE disease and asks that his brain be studied, police sources told ABC News.
The preliminary investigation, according to police, has found no evidence that Tamura suffered from CTE, which medical experts say can't be diagnosed in a living person with certainty.
In a search of the suspect's car, according to authorities, police find additional ammunition, a revolver, a backpack and medication.
July 29 -- Mayor Adams says the preliminary investigation indicates the suspect took the wrong elevator bank to the NFL headquarters and ended up at Rudin Management.
Law enforcement sources tell ABC News that Tamura had two mental health crisis holds in his background in Nevada, one in 2022 and the other in 2024. Those holds typically allow a person to be detained for up to 72 hours if they are thought to be a danger to themselves or others.
President Donald Trump writes on social media that he's been briefed "on the tragic shooting that took place in Manhattan, a place that I know and love."
"I trust our Law Enforcement Agencies to get to the bottom of why this crazed lunatic committed such a senseless act of violence," Trump writes. "My heart is with the families of the four people who were killed, including the NYPD Officer, who made the ultimate sacrifice. God Bless the New York Police Department, and God Bless New York!"