The attorney for the man who sold what would become the deadly weapon in a mass shooting in New York City earlier this week said his client legally sold his co-worker the gun.

The NYPD sent detectives to Las Vegas to work with federal law enforcement in the investigation into Monday's deadly shooting inside a Midtown Manhattan office building.

Four people, including an off-duty New York City police officer, were killed Monday when a gunman carrying a rifle opened fire inside a Midtown Manhattan office tower, authorities said.