UPDATE | 1:50 p.m.
NYPD said Tamura was scheduled to work at Horsehoe on Sunday but he never showed up.
Authorities also said he assembled his rifle using a lower receiver bought by an associate. NYPD said they located the associate and will question him about the purchase as part of a larger operation to trace Tamura's steps from Las Vegas to New York.
VIDEO: Alyssa Bethencourt reports the latest on what we know about the suspect in NYC shooting
Search warrants are being executed on the gunman's car and two cell phones found at the scene, according to authorities.
UPDATE | 11 a.m.
Detectives from NYPD are on their way to Las Vegas as part of the ongoing investigation into the Midtown shooting, New York City Mayor Eric Adams said Tuesday.
Police want to carry out a search warrant and trace the firearms that were recovered, the mayor said.
Detectives found additional ammunition in Shane Tamura’s BMW but it is not known whether he planned to take additional action.
The mayor called it “eerie” to see surveillance video depicting the suspect allowing a woman to walk out of the elevator, sparing her from being shot.
“He allowed her to walk past him without any action at all. He just shot one of the security guards who was behind the desk,” Adams said during an appearance on CNN.
The mayor reported the working theory that the motive appears to be connected to the shooter’s belief he was suffering from CTE and held a grievance against the NFL, for which he never played.
Adams said an autopsy will be conducted by the medical examiner's office. As part of their process, the medical examiner said they will be looking at the gunman's brain.
WATCH: This viewer video captures some of the chaos as the scene unfolded in Midtown Manhattan Monday.
*All videos obtained and shared by ABC News.
ORIGINAL REPORT
We're learning more about the suspected gunman who opened fire at a Manhattan office building and his time in Las Vegas.
Shane Tamura is suspected of killing four people before shooting himself on the 33rd floor of the building at 345 Park Avenue, authorities said.
New York City Mayor Eric Adams said Tuesday that the gunman was attempting to target offices for the National Football League but entered the wrong elevator.
WATCH: Channel 13's Anyssa Bohanan reports live in the neighborhood where Tamura lived as the investigation continues Tuesday morning.
According to the Associated Press, a rambling note found on his body suggested he had a grievance against the NFL over an unsubstantiated claim that he suffered from chronic traumatic encephalopathy (CTE), a degenerative brain disease that has been linked to concussions and other repeated head trauma common in contact sports like football.
We know Tamura attended high school and played football in the Los Angeles area, but did not play professionally; police have found no evidence so far that he suffered a traumatic brain injury or had CTE, sources said. He has no known connection to the NFL, according to a report by ABC News.
According to authorities, surveillance video shows Tamura exiting a black BMW, carrying an M-4 assault rifle.
New York City Police Commissioner Jessica Tisch said he proceeded to enter the commercial building and immediately opened fire. While in the lobby, the suspect allegedly shot and killed a police officer identified as 36-year-old Didarul Islam, who was working as private security. Afterwards, he would go onto the 33rd floor, where he continued opening fire and eventually would shoot and kill himself.
WATCH: New York City authorities share details on Midtown Manhattan shooting
Authorities say Tamura has a Las Vegas address, and the car he was driving was registered in Nevada. Police in New York also discovered medication prescribed to Tamura, ammunition, a loaded revolver, magazines and a backpack.
The car traveled cross-country to New York City.
"The initial investigation shows that Mr. Tamura's vehicle traveled cross-county through Colorado on July 26, then Nebraska and Iowa on July 27, and then in Columbia, New Jersey as recently as 4:24 p.m. today. The vehicle entered New York City shortly thereafter," said Tisch.
New York authorities say that law enforcement in Las Vegas stated that Tamura has a documented mental health history.
Several squad cars from the Las Vegas Metro Police were seen at the entrances to a gated neighborhood where Tamura was believed to have lived.
LVMPD confirmed the department is supporting the New York Police Department with their investigation.
Tamura had two mental health crisis holds here in Nevada, one in 2022 and the other in 2024.
These holds typically allow someone to be detained for up to 72 hours if they're thought to be a danger to themselves or others.
Tamura also had a previous arrest in 2023, and Channel 13 has obtained that report.
According to court documents, Tamura was at Red Rock Casino Resort. When a security officer approached him at a table gambling and asked to see his ID, Tamura refused. He was then asked to leave the property.
Tamura then approached the cashier cage to collect his winnings, but refused to show his ID there as well, documents show. At this point, security called for police assistance.
When officers arrived, they noted Tamura appeared agitated when speaking to security officers. When police asked for his name and birthday, Tamura initially refused to provide that information to authorities. He was told if he didn't he would be placed under arrest, at which time he gave police his name and birth date.
Tamura was told by police multiple times to leave the property, but he refused, leading to the trespassing arrest.
Tamura worked as a surveillance department employee at the Horseshoe hotel and casino, a Horseshoe spokesperson said.
