A jury convicted Mark Jensen last week of killing Julie Jensen on Dec. 3, 1998, in their Pleasant Prairie home. Some jurors cited the letter as a key piece of evidence.
Julie Jensen left the note with a neighbor to give to police if something happened to her.
"I pray that I am wrong and nothing happens, but I am suspicious of Mark's suspicious behaviors and fear for my early demise," Julie Jensen wrote in the letter. She said she refused to leave because of their two young sons.
Mark Jensen, her husband of 14 years, claimed she was depressed, committed suicide and framed him. At the time, Mark Jensen was having an affair with a woman he has since married.
He faces a mandatory penalty of life in prison during sentencing, set for Wednesday. The judge was to determine if he should ever be eligible for parole.
The U.S. Supreme Court will hear a California case with similar elements in April. Legal experts say if the court overturns that conviction, it could pave the way for Mark Jensen to get a new trial.
"It would surprise me if he didn't get a new trial based on that," said Phillip A. Koss, a University of Wisconsin-Madison adjunct professor and Walworth County district attorney.
Mark Jensen, now 48, was charged with first-degree murder in 2002, but legal wrangling over evidence delayed the trial repeatedly.
The evidence included the letter, as well as Julie Jensen's statements to police, a neighbor and her son's teacher about her suspicions.
Until recently, using such evidence in court was virtually unheard of because of constitutional guarantees giving criminal defendants the right to confront their accusers.
But the Wisconsin Supreme Court created new evidence rules, guided by a U.S. Supreme Court decision that laid the groundwork for the use of Julie Jensen's letter and statements to police. The trial judge, Bruce Schroeder, determined last year that the letter and statements should be allowed at trial.
In the California case, Dwayne Giles was convicted in the death of his former girlfriend, Brenda Avie. At Giles' trial, the jury heard statements that Avie made to a police officer a few weeks before her death, describing an assault by Giles and his threat to kill her.
Giles has appealed his conviction, arguing that Avie's statements should not have been allowed because Giles' lawyer never had an opportunity to cross-examine her.
Marquette University Law Professor Dan Blinka said a reversal in the Giles case could help Jensen's appeal. What the U.S. Supreme Court is deciding is "extraordinarily technical," he said.
The panel could decide the evidence should be allowed when the murder is motivated, at least in part, by a desire to silence the witness, or it could allow the evidence regardless of the defendant's motive — how Wisconsin interprets the rule now.
Robert Jambois, who prosecuted the Jensen case, agreed that if the U.S. Supreme Court throws out the California conviction, it will increase the likelihood the Jensen case will have to be retried. But it depends on how broadly the panel rules, he said.
Julie Jensen's brother, Paul Griffin, said the California case worries him after fighting for a decade to get a guilty verdict.
"I'd like to think Mark Jensen is in prison for life and will stay there for life," he said.