A Pennsylvania State University investigation has found no substance behind allegations of academic misconduct by climate researcher Michael Mann, one of the central figures in the so-called 'Climategate' e-mail scandal.

It is the third formal inquiry to clear scientists involved in the scandal, which publicized more than 1,000 private e-mails from scientists expressing doubts about their data, refusing to share information and questioning the work of others.

The Penn State findings, released Thursday by a panel of five senior faculty members, concluded Mann never participated in research or other scholarly activity that "deviated from accepted practices within the academic community."

"We consider this issue closed," said university spokeswoman Lisa Powers.

The e-mails were purloined from a server at the University of East Anglia in Britain, one of the most respected climate research centers in the world, and posted on the Web in November, just before the United Nations climate talks in Copenhagen got underway. Critics seized on the correspondence as evidence scientists, including Mann, were cooking their books to emphasize society's role in climate disruption.

"The scientists have been exonerated," Mann said in an interview. "The science behind human-caused climate change is solid. This really is a problem that we need to confront."

Penn State investigated four charges of misconduct raised again Mann stemming from the e-mails: That he allegedly suppressed or falsified data; deleted or concealed e-mails and data; misused confidential information; and deviated from accepted academic practices.

In January the university cleared Mann of the first three charges but concluded the fourth - academic misconduct - merited further investigation.

The five faculty members conducting the follow-up investigation were unanimous in concluding this fourth charge, too, was unfounded.

"Obviously the topic of climate change and climate research is one of great debate," said Powers. "From our perspective we have done due diligence... Academic misconduct does not happen very often, and when it does, we consider it to be a very serious matter."

The university, Powers added, receives $765 million annually in research money. "We would not put our reputation at risk over a single researcher. Our expectations are very high."

For Mann, a professor of meteorology and director of Penn State's Earth System Science Center, the six-month ordeal was simply a "distraction." It kept him from research and teaching, but it also comes with the territory: Allegations of misconduct and impropriety, he said, will continue to dog climate researchers.

"I don't doubt for a minute that the climate-change deniers will continue their campaign of disinformation and smear. That's all they've got left," he said.

This is not the first time Mann has been down this path.

In 2005, Rep. Joe Barton, R-Texas, called Mann before Congress to testify about his now ubiquitous "hockey-stick" graph, showing temperatures in the Northern Hemisphere over the past millennia, with an abrupt spike upward at the end showing human influence on the climate. The hearing sparked a National Research Council investigation, which confirmed the data behind the graph.

In mid-July a fourth panel, headed by Muir Russell, a retired British civil servant, is expected to release its conclusions on the professional behavior of scientists identified in the Climategate e-mails.

And a separate investigation by Virginia's attorney general into Mann's activities while at the University of Virginia remains underway. Attorney General Ken Cuccinelli questions whether Mann defrauded taxpayers when he accepted five grants worth almost $500,000; he has asked the school to turn over all correspondence between Mann and other scientists. The University of Virginia is fighting the request.

Cuccinelli spokesman Brian Gottstein declined to comment on how Penn State's findings may influence the agency's effort. "All of our commentary will be presented in our July 13 reply brief in court," he said in an e-mail.

Mann is confident these efforts to discredit scientists and undermine climate science will ultimately be judged harshly by history.

"They will continue to attack the science and the scientist," he said. "But I believe that as (the evidence) becomes increasingly compelling, as the public continues to understand that climate change is already unfolding ... we will look back with scorn at those who denied climate change."

"All they did, by muddying the water and confusing the public, is delay action to the point where necessary work to mitigate the effects is more expensive."