The 36-year-old gunman who killed seven people and wounded 22 others in West Texas on Saturday purchased his AR-style rifle from a private seller, federal law enforcement sources confirmed to CBS News. The purchase allowed the gunman to evade a federal background check. Sources also say the gunman was previously denied a gun purchase because he was determined to be mentally unfit.

The investigation into who sold the weapon is ongoing. Authorities said the gunman was killed by officers outside a busy movie theater in Odessa. It was at least the 38th mass killing this year.

Online court records show the gunman, identified as Seth Aaron Ator, was arrested in 2001 for a misdemeanor offense that would not have prevented him from legally purchasing firearms in Texas. Federal law defines nine categories that would legally prevent a person from owning a gun, which include being convicted of a felony, a misdemeanor domestic violence charge, being adjudicated as a "mental defect" or committed to a mental institution, the subject of a restraining order or having an active warrant. Authorities have said Ator had no active warrants at the time of the shooting.

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FBI special agent Christopher Combs said Ator "was on a long spiral down" before the shooting on the day he was fired from his trucking job. He went to work that day "in trouble," Combs said. He said the place where the gunman lived was "a strange residence" and that the condition reflected "what his mental state was going into this."

Combs said Monday that Ator called the agency's tip line as well as local police dispatch Saturday after being fired from Journey Oilfield Services, making "rambling statements about some of the atrocities that he felt that he had gone through."

"He didn't wake up Saturday morning and walk into his company and then it happened. He went to that company in trouble," Ator said.

Fifteen minutes after the call to the FBI, Combs said, a Texas state trooper unaware of the calls to authorities tried pulling over Ator for failing to signal a lane change. That was when Ator pointed an AR-style rifle toward the rear window of his car and fired on the trooper, starting a terrifying police chase as Ator sprayed bullets into passing cars, shopping plazas and killed a U.S. Postal Service employee while hijacking her mail truck.

Police cars and tape block off a crime scene where the gunman was shot and killed on August 31, 2019, in Odessa, Texas. Getty

The daylight attack over the Labor Day holiday weekend came just weeks after another mass shooting killed 22 people in the Texas border city of El Paso.

Texas Governor Greg Abbott tweeted Monday that "we must keep guns out of criminals' hands" — words similar to his remarks that followed the El Paso shooting August 3, when he said firearms must be kept from "deranged killers." But Abbott, a Republican and avid gun rights supporter, has been noncommittal about tightening Texas gun laws.

Odessa Police Chief Michael Gerke said Ator's company also called 911 on Saturday after Ator was fired but that Ator had already taken off by the time police showed up.

"Basically, they were complaining on each other because they had a disagreement over the firing," Gerke said.

Authorities said they remain unable to provide an exact timeline of the shooting, including how much time passed between the traffic stop at 3:13 p.m. and police killing Ator at the movie theater.

Jeff Pegues contributed to this report.