The mass shooting at a Manhattan office building raises new concerns for business leaders looking to secure their staff and buildings from threats.

The funeral will be held Thursday for the police officer who was one of four people killed in Monday's mass shooting at a Midtown Manhattan office building.

Officials said the gunman fatally shot himself after his attack, which set off a chaotic scene that rippled out into the heart of Manhattan during rush hour.

This is a breaking news story. For latest updates, please go to our live blog. An NYPD officer and at least four other victims were killed in a shooting at an office skyscraper in midtown Manhattan Monday evening,

The gunman accused of killing four people in a Midtown Manhattan office tower bought the AR-15-style rifle used in the attack from his boss at a Las Vegas casino.

A gunman who killed four people at a Manhattan office building bought the rifle from his supervisor at a Las Vegas casino, authorities said.

Shane Tamura, 27, who police said fatally shot four people Monday in the midtown skyscraper that houses the National Football League's headquarters, said in a suicide note that he believed he had chronic traumatic encephalopathy,