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.

Investigators are combing through evidence in the wake of New York City’s deadliest mass shooting since 2000, trying to figure out more about the gunman who unleashed an assault-style rifle in Midtown,

Officials said the gunman fatally shot himself after his attack, which set off a chaotic scene that rippled out into the heart of Manhattan during rush hour.