The gunman who opened fire at a Manhattan office building Monday, killing four people before taking his own life, legally purchased the rifle used in the attack, according to an attorney for the Las Vegas casino supervisor he bought it from.

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 New York City Police Department sent detectives to Las Vegas to work with the Bureau of Alcohol, Tobacco and Firearms to execute search warrants and continue the investigation into Monday's deadly shooting inside a Midtown Manhattan office building.