LOUISVILLE, Ky. -- Boxing champ Muhammad Ali is asking Iran to release two American hikers held since 2009 on spy charges.

Ali, arguably the most prominent U.S. Muslim, on Wednesday released to the press a letter he wrote to supreme leader Ayatollah Ali Khamenei in February.

The letter asks Khameini to release Josh Fattal and Shane Bauer, who were arrested while hiking in northern Iraq near the Iranian border. A third hiker, Sarah Shourd, was released on bail in September.

"Please show the world the compassion I know you have in your heart," he wrote, asking Khamenei as a brother in Islam to show the same mercy and compassion for the two men as he did for Shourd.

Ali also wrote a letter to Khamenei shortly before Shourd was released, though it was unclear whether it had any effect.

The 69-year-old is the founder of a center for world peace in Louisville, where he grew up and launched a boxing career that included three world heavyweight titles.

His wife, Lonnie Ali, told the press in a telephone interview from their Arizona home that her husband was asked to intervene by the late John Arum, son of boxing promoter Bob Arum.

John Arum, also a hiker, died in a climbing accident on Storm King Mountain in Washington state shortly after he asked Ali to help and before Shourd was released, she said.

"We felt we could at least send a letter on their behalf," Lonnie Ali said. "So that's what we did and we followed up with this letter."

She said they did not receive any responses to the letters.

Lonnie Ali said her husband has visited Iran twice before, including a trip in the early 1990s where he tried to secure the exchange of prisoners during the Iran-Iraq war.

She said he would be willing to go to Iran to help secure the hikers' release, but it would depend on his health. Parkinson's disease has limited his speech and physical activity.

Lonnie Ali said Alex Fattal, Josh Fattal's brother, had visited the Alis in Arizona on Tuesday. It was their first meeting.

The trial for the three Americans began in February, and they pleaded not guilty to the espionage charges. Shourd pleaded not guilty in absentia.

The second session of the trial is scheduled for May 11 in Tehran.

The U.S. government has denied the charges against the hikers and demanded their release. Their lengthy detention has added to tensions between the two nations over issues such as Iran's disputed nuclear program.