Investigators worked Monday to determine why a 45-year-old painter busted into a North Carolina nursing home on a horrific shooting rampage that left eight dead and several others injured.

One possible clue: the gunman's estranged wife works at the 110-room Pinelake Health and Rehab, where authorities say Robert Stewart shot and killed seven residents — most in their late 80s — and a nurse who cared for them.

"We're certainly looking into the fact that it may be domestic-related," said Carthage Police Chief Chris McKenzie.

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McKenzie said Monday he believed the couple were recently separated, but acknowledged he didn't yet know many more details about her. A neighbor said Stewart's estranged wife is a certified nurse assistant at the nursing home named Wanda Luck, and public records indicate she shared an address with Stewart in Carthage.

Several telephone numbers for Luck or her family were disconnected. A knock at the door was unanswered at Luck's address.

"This is a horrible tragedy," said Mark Barnett, the neighbor. "This is something that's just unthinkable. I can't even imagine what she's going through."

Moore County District Attorney Maureen Krueger said information on a motive for the shooting spree was incomplete.

An ex-wife of Stewart's, Sue Griffin, told reporters Sunday she had not had contact with him since they divorced in 2001, but said he had been recently reaching out to family members, telling them he had cancer and was preparing for a long trip and to "go away." Griffin added she didn't know how her ex-husband was connected to the nursing home or why he would shoot people there.

"He did have some violent tendencies from time to time," Griffin said. "I wouldn't put it past him. I hate to say it, but it is true."

Authorities said the attack began around 10 a.m. Sunday, when Stewart arrived at the nursing home and care center for patients with Alzheimer's disease and started firing.

McKenzie said he was armed with more than one weapon, and witnesses said he was shooting a "deer gun." Police recovered a camouflaged-colored rifle or shotgun from the home's parking lot, where at least two cars had their windows shattered, apparently by gunfire.

McKenzie said the massacre would have been much worse if not for a single shot fired by a decorated police officer Justin Garner, 25, who was Carthage's only police officer on duty at the time of the shooting. Garner shot Stewart in the upper torso while the two traded gunfire in a hallway. Garner himself was shot three times in the leg.

I can’t classify it as anything other than heroic," McKenzie said. "If that’s not heroism, I don’t know what is. A lot more lives would have been lost if he hadn’t done what he did."

He added: "Whether he realizes it now, he will hopefully realize someday how many lives he has saved."

Stewart was not scheduled to appear in court until next week on eight counts of first-degree murder and a charge of felony assault of a law enforcement officer. McKenzie said he did not know if Stewart had an attorney.

Detectives were working at the state medical examiner's office in Chapel Hill on Monday to recover evidence, and search warrants police executed in the hours after the shooting were sealed. Court records said Stewart was transferred from the custody of the Moore County Sheriff to the state Department of Correction, because he has a gunshot wound.

Krueger said the victims were Pinelake residents whose ages ranged from 78 to 98 years old as well as a male nurse whose age wasn't immediately available.

Sunday's rampage in North Carolina's Sandhills region, about 60 miles southwest of Raleigh, happened just weeks after a man killed 10 people, including his mother and several other relatives, in the worst mass shooting in Alabama's history on March 10.

The Associated Press contributed to this report.