The dry summer heat had already begun to settle over Las Vegas as the man left his third-floor apartment at the Paradise Royale.

It was still early on July 26 as he descended the open stairs and walked past the window of his neighbor, Michael Valentic. On most days, Mr. Valentic — a retiree who lived by the clock — would walk to the parking lot at 6:37 a.m. and see the man in a black BMW, returning from an overnight security guard shift at the Horseshoe Casino.

Today the man was leaving, not arriving, and Mr. Valentic thought he looked like he had somewhere to be: He was toting several packs and shouldering a bulky canvas bag. The man stepped into the courtyard before rounding the corner, out of sight.

Two days later, news updates flashed on Mr. Valentic’s computer screen in quick succession. There had been an attack at a Manhattan office tower; the gunman had entered the high-rise undetected; he had driven from Las Vegas to New York; he had fatally shot four people before killing himself.

Within hours, Mr. Valentic saw a surveillance photo of the gunman strutting toward 345 Park Avenue, an assault rifle dangling at his side. Mr. Valentic gasped.

The killer was the man who lived upstairs.

In the weeks since the shooting, interviews with neighbors, a colleague, a landlord and former housemates, as well as public records and police documents from New York and Las Vegas, have revealed that the man, 27-year-old Shane Tamura, had been struggling for years, a worry to his family and sometimes a problem for the police.