Jurors were dismissed after telling a federal judge they could not reach a consensus on 15 counts of securities fraud and two counts of bank fraud. Rigas was acquitted of conspiracy and wire fraud charges a day earlier.
"There's no one who questions for a moment that you tried," U.S. District Judge Leonard Sand told the jury. "Of course we're all disappointed that you have not been able to reach a unanimous verdict as to Michael Rigas."
The mistrial came a day after Rigas' father, 79-year-old Adelphia founder John Rigas (search), and his brother, former chief financial officer Timothy Rigas, were convicted of conspiracy, bank fraud and securities fraud.
A fourth defendant, Michael Mulcahey, the cable company's former assistant treasurer, was acquitted of all charges on Thursday.
Sand said a retrial for Michael Rigas could happen as early as fall. U.S. Attorney David Kelley said outside court that prosecutors would "adhere to the court's schedule" — an apparent indication the government plans to retry the case.
Michael Rigas did not address reporters as he left the courthouse. His lawyer, Andrew Levander, said: "It's a very good result for us. Unfortunately, the government may retry the case."
Adelphia, then based in tiny Coudersport, Pa., collapsed into bankruptcy in 2002 after the company disclosed $2.3 billion in off-balance-sheet debt. It now operates under bankruptcy protection in Greenwood Village, Colo.
At the trial, Michael Rigas' lawyer tried to portray him as distant from the financial decisions of the company and without much of a taste for personal luxury.
The description stood in contrast to testimony about John Rigas, who was described by one witness as so willing to take cash from the company that Timothy Rigas had to limit his withdrawals to $1 million per month.
Most jurors left the courthouse together and did not take questions from reporters.
Elizabeth Carballo, who sat for months as a juror and was taken off the case when she broke her ankle around the time of closing arguments, watched from the audience as the jury was dismissed.
She said she was not certain whether she would have convicted the four defendants. Asked whether she felt sorry for the Rigases, she said: "I'm a human person. I feel bad when things happen to people."
The panel had been sitting for more than four months. The trial included a blizzard of paper evidence, including documents reflecting complex loan arrangements and cable-subscriber statistics.
The judge discharged jurors after a lengthy speech in which he thanked them for sitting patiently and attentively through the trial.
"This is not a simple case of, `Did A hit B?' or `Who was responsible for the automobile accident?'" Sand said. "It involved many issues and concepts that you don't encounter in your daily life."
The judge smiled and added, "Now, you're going to miss us, right?"
Prosecutors had already notched two major victories by winning convictions against John Rigas, who founded the company in 1952, and Timothy Rigas.
Jurors had said on Thursday they were still undecided on one technical aspect of the conspiracy count against John and Timothy Rigas — whether the pair conspired to falsify company books and records.
But they found the two conspired to commit other crimes, and prosecutors agreed Friday not to seek resolution of the remaining part of the count, meaning John and Timothy Rigas did not have to sit for further deliberations.
Speaking briefly with reporters before he left court, John Rigas said the verdict Thursday was "something I never thought would end up the way it did." He also said he was worried about his blood pressure.
John and Timothy Rigas each face 30 years in prison on the bank fraud count alone.
The judge set a Sept. 21 hearing for post-trial issues, including how to handle a recent Supreme Court decision that some legal experts say calls into question the federal sentencing guidelines.
The decision found that Washington state judges may not increase sentences beyond the guidelines based on facts that have not been proved to a jury beyond a reasonable doubt.
Legal experts have said the decision could force prosecutors to include information intended for sentencing during the trials themselves.
Sand told prosecutors they should consider filing a new indictment of Michael Rigas to make it "Blakely-compliant" — a reference to the Supreme Court decision.