New York City Mayor Eric Adams shared new details about Monday's deadly mass shooting at a Manhattan office building that left four people dead and one injured, describing the incident as an "intentional, sick, twisted act of violence."

In an exclusive interview with ABC News' Aaron Katersky, Adams said the gunman, who drove cross-country from Nevada, appeared to target the NFL headquarters located in the building at 345 Park Avenue.

A note left by the shooter mentioned CTE (chronic traumatic encephalopathy), though investigators were still working to determine the exact motive.

"We're far from the investigation to finalize exactly what happened," Adams told ABC News. "We're looking at his note that talked about CTE... but it is far from conclusive."

The mayor confirmed the shooter had played high school football, not at the college or professional level.

Security camera footage showed the gunman's methodical approach, Adams said.

"When you look at the tapes... there was just a total disregard for human lives," Adams said.

The shooter allowed a woman to walk past him unharmed, while he had attacked everyone else he encountered. Among those killed were Wesley LePatner, a Blackstone employee and mother of two; Didarul Islam, an off-duty NYPD officer; security officer Aland Etienne; and Julia Hyman, a Rudin Management employee. An NFL employee was also injured in the attack.

Adams praised the building management's security measures, including having an armed security officer, emergency protocols and active shooter drills.

"I believe their actions saved the lives of many employees," he said.

Jessica Chen, who was in the building during the shooting, described the terrifying scene to ABC News.

"We heard multiple shots go off in quick succession from the first floor," she said. Chen and about 150 others barricaded themselves in a second-floor conference room.

"I texted my parents that I loved them," Chen told ABC News. "Nothing could have prepared any of us. I think all of us were frozen."

The mayor emphasized the need for stricter gun control measures nationwide as he stands on New York City being the safest big city in America.

"The response to shooters of this magnitude, mass shooters, cannot be vigils. It must be legislation," Adams said, questioning the availability of automatic weapons.

Two teams of detectives were sent to Las Vegas to execute a search warrant on the shooter's home and investigate the weapons used in the attack. Investigators discovered that parts of the automatic weapon had been purchased by someone else, raising questions about potential assistance in the crime.

Mayor Adams told ABC News that President Donald Trump called him and expressed his condolences and praised the NYPD's response. Adams commended the officers who entered the building during the active shooter situation.

"They went into the building with knowledge that there was a shooter present," Adams said. "They did not wait, and they knew their timing was important."