In a speech at Australia's University of Adelaide, Justice Stefan Lindskog, chairman of the Supreme Court of Sweden, listed legal obstacles to extraditing the 41-year-old Australian to the United States to face prosecution for exposing thousands of classified documents.
Assange has taken asylum in the Ecuadorean Embassy in London since last June to avoid extradition to Sweden on sex crime allegations.
He is wanted in Sweden for questioning over criminal allegations made by two women. But Assange says the Swedish allegations are a ploy to get him to Sweden from where he would be extradited to the United States.
The U.S. Department of Justice has been investigating WikiLeaks since the secret-busting website began distributing hundreds of thousands of classified U.S. documents. But few details of that investigation have been made public.
Lindskog was critical of the Swedish criminal investigation.
"I think it is a mess," he said.
"Basically, I think there are some misunderstandings, especially when it comes to the issue of extradition," he added, without elaborating.
Lindskog suggested that Sweden's extradition treaty with the United States would not apply to Assange.
"Extradition shall not be granted when alleged crimes (are) military or political in nature," he said.
U.S. soldier Bradley Manning last month admitted sending Iraq and Afghanistan battlefield reports, State Department diplomatic cables, other classified records and two battlefield video clips to WikiLeaks in 2009 and 2010. WikiLeaks posted some of the material, embarrassing the U.S. and its allies.
Assange has refused to say whether he had any dealings with Manning, but he called him a political prisoner and said his prosecution was part of an effort by the U.S. to clamp down on criticism of its military and foreign policy.
Lindskog praised Assange's public information campaign.
"He'll be thought of as a person who made public some pieces of classified information to the benefit of mankind," he said.
"It should never be a crime to make known (a) crime of a state," he added.