ANCHORAGE, Alaska -- A 91-year-old Anchorage man has been charged with first and second-degree murder in the shooting death of his 92-year-old wife.

Duane Marvin also is charged with evidence tampering in the death of Charlotte Marvin.

Anchorage police say they don't know the motive for the shooting. According to an Anchorage Police charging document obtained by the Alaska Dispatch News, Duane Marvin told investigators: "I killed my wife and I don't know why."

"I don't know what happened," and "I had no reason to do anything to my wife" were among the statements he gave investigators, the paper reports.

Police went to the couple's house Sunday afternoon after receiving calls that a woman was dead in her home on east 28th Avenue.

Investigators concluded that Duane Marvin shot Charlotte Marvin, hid the gun in a cabinet and walked to a neighbor's home to report his wife's death.

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Police say they saw blood spatter on the front wheel of Duane Marvin's walker and a red mark between his thumb and pointer finger consistent with the slide of a pistol pinching skin, the paper reports. Investigators later seized the man's clothing and reportedly found three 9mm bullets in his pants pocket.

Police on Monday afternoon obtained an arrest warrant for Marvin. He turned himself in just after 5 p.m.

Marvin was arraigned Monday and a judge approved his release, appointing his granddaughter Gena Powers as his custodian, reports the Alaska Dispatch News.

Powers told a judge he has dementia -- though she didn't believe he took medication for it --- as well as hearing loss, and trouble walking and communicating. Powers said he is a decorated WWII veteran.

Marvin's defense attorney, Jon Buchholdt, says the shooting was an accident, and the gun went off because he fell, reports CBS affiliate KTVA.

"He stumbled in a bedroom with his wife, and he's crippled, and he fired off a gun because he's a gun freak," said Buchholdt.

Prosecutors argued evidence proved otherwise, and said there was no age limit when it comes to the law, KTVA reports.

Marvin was pushed to the court podium in a wheelchair and cried through much of the hearing, the paper reports. At one point, he reportedly said, "I killed my wife."