(CNN) -- A jury in Wisconsin found Mark Jensen guilty of first-degree murder Thursday in the 1998 poisoning death of his wife after a trial that included what prosecutors said was a haunting letter from the grave.
Defense attorney Craig Albree advises Mark Jensen after the juryfinds him guilty.
The jury of seven women and five men deliberated for nearly 32 hours over three days.
Jensen sat stonefaced at the defense table as the jury was polled. He showed little emotion during the six-week trial.
His bail was immediately revoked. Attorneys will return to court Friday to set a date for sentencing. Watch the drama in the courtroom »
The conviction carries a mandatory life prison sentence. Whether Jensen may some day be eligible for parole will be decided at the sentencing hearing.
The jury's verdict came nearly a decade after Julie Jensen was found dead in her bed. The cause of death: Poisoning by ethylene glycol, the main ingredient in antifreeze.
"I think what Mark Jensen did is the most unspeakable offense I can truly recall," prosecutor Robert Jambois said after the verdict. "That's one of the reasons it took so long to bring this case to justice. It took a long time to uncover the lies and the machinations of Mark Jensen."
Julie Jensen, the victim, had given a neighbor a letter pointing an accusing finger at her husband should anything happen to her.
She also made foreboding comments to police and to her son's teacher, saying she suspected that her husband was trying to kill her.
Her letter, read aloud in court, said in part: "I pray I'm wrong + nothing happens ... but I am suspicious of Mark's suspicious behaviors + fear for my early demise," the letter says. " Read the letter »
After the verdict, jurors told reporters that the letter gave them "a clear road map" to conviction, as one female juror phrased it.
Another female juror said he believed Mark Jensen was trying to push his wife over the edge. "He tortured Julie hoping she could be classically diagnosed as a nutcase," she said. Watch jurors discuss 'road map to murder' »
"Julie's letter, we took it as a mandate, and we passed that mandate on to the people of Wisconsin," said Julie Jensen's brother, Larry Griffin, after the verdict.
Mark Jensen, 48, was charged in 2002 with first-degree murder in the December 1998 death of his 40-year-old wife.
Prosecutors alleged that Jensen was having an affair and poisoned his wife so he could be free of her. The defense says Julie Jensen was despondent about the affair, killed herself and tried to frame her husband.
The testimony during the six-week trial has been dramatic. It included evidence of his-and-hers flings, X-rated e-mail exchanges and Internet searches for poisons. Prosecutors also presented testimony from Jensen's new wife, Kelly, and from a jailhouse snitch who said Jensen had made incriminating remarks behind bars.
The drama lasted up to the final moment of testimony, when a prosecution forensics expert dipped her fingers into a Styrofoam cup of antifreeze, tasted it and described the flavor as "sweet."
Jensen did not take the stand, and his defense relied primarily on testimony from forensic and mental health experts.
Legal wrangling over the letter and Julie Jensen's statements delayed the trial for years.
Using such evidence in court has for years been blocked by strict hearsay rules giving criminal defendants the right to confront their accusers.
But the Wisconsin Supreme Court, guided by a U.S. Supreme Court ruling, created a hearsay exception that permitted the use of Julie Jensen's letter and statements as a dying declaration -- evidence of her state of mind at the time of her death.
For years, authorities said Julie Jensen had died of multiple doses of ethylene glycol, commonly used as antifreeze. But testimony during the trial also indicated that she might have been smothered with her pillow.
Jurors said they could not decide whether or not Julie Jensen suffocated, but concluded she had died from ethylene glycol poisoning.
Inmate Aaron Dilliard, an admitted con man, testified that Mark Jensen indicated to him that he suffocated his wife when the poison did not appear to be working fast enough.
Dillard testified that Jensen said he sat on Julie's back and pushed her face into the pillow. Crime scene photos show Julie's nose and mouth pushed to the left side. Her face was found deep in the pillow, according to testimony.
Another inmate, bank robber David Thompson, testified that Jensen told him last year that he killed his wife and asked him to help kidnap and "sit on" a witness until after the trial.
The conversation was overheard by a third inmate, Bernard Bush. Bush said he heard a total figure of $100,000 being discussed, with $50,000 up front and $50,000 at the completion of the abduction.
The would-be target was Ed Klug, Jensen's former co-worker. He testified that Jensen told him he thought about poisoning his wife. During a night of drinking in November 1998, Klug said, Jensen revealed that he was researching ways to do away with her.
Prosecutor Robert Jambois called experts who say they found evidence of suffocation. The defense experts disagreed.
Defense attorney Craig Albee called his own poison expert to say Julie Jensen could have taken repeated doses of poison herself, contradicting the prosecution's poison expert.
The defense also called to the stand the Jensen family doctor, who said he saw Julie Jensen a few days before her death and who described her as depressed and frantic. E-mail to a friend
CNN's Gary Tuchman and In Session's Jean Casarez contributed to this report.