JEFFERSON CITY, Mo. (AP) — The legal fate of a northwest Missouri man appealing his murder conviction in a neighbor's 1990 death hinges on whether the state Supreme Court agrees that prosecutors withheld potentially helpful evidence.

The Missouri Supreme Court held oral arguments Thursday in the case of Mark Woodworth, who was 16 when Cathy Robertson— the wife of his father's farming partner — was shot to death while sleeping in her home near Chillicothe.

He was first convicted in 1995, and a second jury convicted him four years later after briefly being released on appeal. Woodworth is serving a life sentence.

His attorney alleged Thursday that the conviction was ''tainted'' by prosecutors' failure to turn over copies of letters that cast doubt on Woodworth's guilt, as well as other evidence that would have benefitted the defense. The letters — between a Livingston County judge, state and local prosecutors and the victim's husband, Lyndel Robertson, who was also shot during the attack but survived — were first publicly disclosed by The Associated Press in 2009.

Those missing letters were among the investigative and prosecutorial flaws cited by a Boone County judge who determined in May that Woodworth was the victim of ''a manifest injustice'' and deserves to be freed or receive a third trial.

''The defense was deprived of very powerful evidence that these investigative officers were engaged in a pattern (of deceit),'' attorney Robert Ramsey told the Supreme Court justices Thursday, outlining what he called numerous conflicts of interest, including designating a private detective who was hired by Robertson's husband as the Livingston County Sheriff's Office's de facto lead investigator.

Assistant Attorney General Ted Bruce countered that despite the May ruling, Ramsey failed to prove that Woodworth's previous defense lawyers had not been provided those letters and other disputed evidence.

Even if they had, the documents cited by Ramsey don't contain any significant new information that would have changed the trial's outcome or not led to Woodworth's arrest, he said.

''There is no new evidence in this case,'' he said. ''Mr. Woodworth has not sustained the burden of his proof.''

Woodworth did not attend the hearing, and the court offered no details on when it expects to reach a decision.

More than two decades later, the case continues to intrigue Chillicothe, which is located about 120 miles northwest of Columbia. The usually quiet hearing room was packed with Woodworth supporters and Robertson allies, including the couple's five adult children. A nearby courtroom held the overflow and showed the 40-minute proceeding on video.

The prosecutor at Woodworth's first trial was Kenny Hulshof, who served six terms in Congress but whose career as a special state prosecutor was marked by a pattern of court rulings that questioned his courtroom behavior. Two men he helped convict for murder have since been released after judges cited prosecutorial misconduct.

Both Bruce and a Robertson family spokeswoman said that Woodworth's appeal focused too heavily on perceived shortcomings in the investigation and first trial at the expense of the evidence used to convict Woodworth.

From his hospital bed, Lyndel Robertson initially identified his oldest daughter's ex-boyfriend, Brandon Hagan, as the likely shooter, according to court records. But he later testified that he only named Hagan, who has denied involvement, as a possible suspect.

In May, Circuit Judge Gary Oxenhandler recommended that the convictions be set aside, but the Supreme Court will have the final say. Ramsey asked the justices to consider freeing his client outright under an actual innocence claim.

Ryan said that Attorney General Chris Koster has assured the Robertson family he will try Woodworth a third time if the high court throws out the conviction. A Koster spokeswoman confirmed that commitment.

Questions by the five justices at the hearing (a sixth did not attend) suggested that Oxenhandler's conclusions will weigh heavily in their decision. Bruce, though, said that Oxenhandler ''was encouraged to speculate'' about the relevance of certain claims by Woodworth.