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

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,