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.

New York City police detectives searched the Las Vegas studio apartment of the suspected gunman in Monday's mass shooting at a Midtown Manhattan office tower and found what law enforcement sources described to ABC News as a suicide note he left for his parents.

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.