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 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.

A man carrying a rifle killed four people at a New York City office tower Monday, including an off-duty New York City police officer, and wounded a fifth before taking his own life