The shooter inexpicably let a woman pass unharmed during his rampage on Monday night.
Shane Temura had opened fire inside the lobby of 345 Park Avenue, and after he shot three people he called for an elevator to the 33rd floor.
At that point, Police Commissioner Jessica Tisch said, a woman left the elevator shaft and Temura let her pass unharmed before he made his way upstairs.
There, another woman was found dead, as was Temura of a self-inflicted gunshot wound to his chest.
Gunman Shane Tamura traveled across the country from his home in Las Vegas to New York City to carry out the attack, Police Commissioner Jessica Tisch announced at the news conference.
His vehicle was seen traveling through Colorado on Jul 26 and passed through New Jersey earlier on Monday evening.
The black BMW was searched, and police found a backpack with a rifle and ammunition, as well as medication prescribed to Tamura.
The gunman had known mental health issues, Tisch said.
NFL employees were urged to 'hide' after an active shooter stormed into the building which includes the league's headquarters in New York on Monday.
According to
ESPN's Jeff Darlington, an NFL security alert sent to employees read: 'Do not exit the building. Secure your location and hide until law enforcement clears your floor. Please switch phones to silent.'
Late on Monday, a gunman - seen carrying an assault rifle - killed four people, including one police officer, after he opened fire in broad daylight in the heart of New York City, before he reportedly took his own life.