The report said a family grudge was a likely factor in the dismissal
The running mate of US Republican presidential candidate John McCain has been found guilty of an abuse of power, according to a state legislature probe.
Alaska's Governor Sarah Palin was accused of sacking a senior state official over a family feud.
But the McCain-Palin campaign team said that the report showed Mrs Palin acted within "proper and lawful authority".
The report could have an impact on Republican hopes of winning next month's US presidential election.
"I find that Governor Sarah Palin abused her power by violating Alaska Statute 39.52.110 (a) of the Alaska Executive Branch Ethics Act," investigator Steve Branchflower concluded in the panel's 263-page report.
Sarah Palin abused her power
Monegan's refusal to fire Wooten was not sole reason for his dismissal, but a contributing factor
Sarah Palin's acted within her rights as Governor of Alaska
Mrs Palin has always denied any wrongdoing, and her supporters say the charges were motivated by her political opponents.
"This was a partisan-led inquiry run by [Democratic candidate Barack] Obama supporters and the Palins were completely justified in their concern," a McCain campaign spokesperson said.
Mrs Palin was accused of dismissing Mr Monegan for refusing to sack a state trooper who was in a bitter custody battle with her sister.
The report concluded a family grudge was not the sole reason for the dismissal, but was a likely contributing factor.
However, the report said that the actual sacking of Mr Monegan was not beyond Mrs Palin's legal powers.
Speaking after a bipartisan investigating panel reached its decision on what has become known as Troopergate, Mr Monegan said he felt "vindicated".
"It sounds like they've validated my belief and opinions," he said. "And that tells me I'm not totally out in left field."
The panel found Mrs Palin in violation of a state ethics law prohibiting public officials from using their office for personal gain.
Legislators do not have the power to take formal legal action against the governor; that would be up to Alaska's Personnel Board.
If the Board decides Mrs Palin violated state law, the case will be referred to the president of the state Senate.
Mrs Palin's lawyer said that the report had not been conclusive.
"In order to violate the ethics law, there has to be some personal gain," said Thomas Van Flein.
"Mr Branchflower has failed to identify any financial gain."
And Alaskan state Senator Gary Stevens, a Republican, said there were "some problems" with the finding.
Palin's supporters say the probe was politically motivated
"I would encourage people to be very cautious, to look at this with a jaundiced eye," said Senator Stevens, after the report's release was announced.
Several Republican politicians had earlier attempted to have the investigation stopped on the grounds that it was politically motivated.
The investigation into the affair began before Mr McCain selected Mrs Palin as his running mate in August.
The US presidential race has now become so polarised both Republicans and Democrats will likely see the report's findings as vindication for their own trenchant views about Mrs Palin, says the BBC's Richard Lister in Washington.
Alaska's governor will either be seen as the victim of a Democratic party hatchet job, or a hypocrite.
Most voters, for now at least, seem more concerned about who will extract them from the current economic crisis, rather than any questions about political infighting in far-off Alaska, our correspondent adds.
Mrs Palin maintains she fired Mr Monegan in July over a budgetary dispute.
But Mr Monegan said he was dismissed for resisting pressure from Mrs Palin and her husband, Todd, to fire State Trooper Mike Wooten, Mrs Palin's former brother-in-law.
Mr Monegan said he simply wanted the truth to be made known.
Sarah Palin has denied any wrongdoing over the affair
"The governor did want me to fire [Mr Wooten], and I chose to not," he told the Associated Press news agency.
"He didn't do anything under my watch to result in termination."
Todd Palin has admitted he did publicise what he called the "injustice of a violent trooper keeping his badge".
But he said his wife, who did not give evidence to the enquiry, then told him to drop the matter.