ODESSA, Tex. — Monette Rodriguez remembers the oil field truck-driver who lived at the Peppertree Place Apartments. He had the phrase “Oilfield Mafia” displayed on the back window of his blue Dodge Challenger. And he liked to party — neighbors complained that women made frequent visits at all hours of the night to his apartment.

The truck driver was Seth A. Ator, 36, the gunman who waged a mobile mass shooting that spread panic and bloodshed across the West Texas sister cities of Midland and Odessa on Saturday. Ms. Rodriguez, 41, a former Peppertree Place manager, said the gunman used to live in the complex several years ago. She said he was never disrespectful to her, but she recalled walking into his apartment after he had moved out and finding a large amount of pornographic material he had left behind.

“That, to me, was very disturbing,” Ms. Rodriguez said.

On Monday, the depth of the gunman’s erratic and nomadic life and behavior became clearer, as the authorities described how he was fired from his trucking job the morning of the shooting, called the F.B.I. tip line and was even on the phone with 911 dispatchers as he carried out the attack. He had been living a kind of drifter’s life in the West Texas oil fields, estranged from many relatives, grappling with the suicide of his older sister and ditching apartment life for a secluded shack-style building where he shot his firearms outside late into the night.