SALT LAKE CITY — A Utah doctor convicted of killing his wife in a case that became a true-crime cable TV obsession is appealing his conviction by saying key testimony in the case was tainted.

Lawyers for Martin MacNeill laid out their case for tossing his conviction in court documents obtained Wednesday by the Associated Press.

An ex-cellmate who testified that MacNeill had confessed was the only person who could directly tie him to the crime in the largely circumstantial case, according to attorney Kent Morgan. But the jury never heard evidence that the inmate had seen cable news coverage of the trial, or that prosecutors recommended early release in exchange for his testimony.

"Whether the jury believed or disbelieved this informant was the linchpin in the prosecution's case," Morgan wrote in court papers.

Defense attorneys requested oral arguments in the case, though no date was immediately set for the proceeding.

Prosecutors didn't immediately return messages Wednesday seeking comment.

MacNeill, 59, was sentenced to up to life in prison after he was convicted of dosing his wife with drugs prescribed after cosmetic surgery and leaving her to drown in the bathtub so he could begin a new life with his mistress.

He maintained his innocence, and his attorneys say the woman likely died of natural causes, possibly a heart attack.

The case shocked the Mormon community of Pleasant Grove, 35 miles south of Salt Lake City. It captured national attention because the defendant was a wealthy doctor and a lawyer, a father of eight in a picture-perfect family and former bishop in his local congregation of The Church of Jesus Christ of Latter-day Saints.

The trial came after his daughters and other relatives were unconvinced by an initial finding that Michele MacNeill's 2007 death was from natural causes. They doggedly pursued authorities for years to keep investigating after Martin MacNeill introduced mistress Gypsy Willis as a nanny within weeks of his wife's death.

Prosecutors filed charges of murder and obstruction of justice in 2012, when Martin MacNeill completed a federal prison sentence for fraud.

In the appeal documents filed Friday, defense attorneys call it "asinine" to suggest MacNeill must be a murderer simply because his family believed he was immoral.

Defense attorneys have raised questions about the cellmate's testimony before, but Judge Derek Pullan decided that jurors didn't put much stock in it after a withering cross-examination from the defense at trial.

Jurors said publicly they were swayed by MacNeill's behavior after her death and the culmination of evidence, not primarily by allegations from a handful of men who spent time with MacNeill behind bars. They said he acknowledged drugging and drowning his wife, or bragged that investigators could never prove it.

MacNeill gave notice that he would appeal in November and filed the opening brief in the case Friday.

The 2013 verdict in the case was troubling to some in the legal community who pointed out that medical examiners were never able to rule the death a homicide and prosecutors built a case largely on circumstantial evidence. Others, though, said the evidence stacked up and the jury did its job.