Following a sacred pipe ceremony Thursday morning at a park in south Minneapolis, about 20 activists from Canada and the United States set off on a 1,100-mile walk to Washington, D.C., to seek what organizers hope will be justice for imprisoned American Indian Movement (AIM) member Leonard Peltier.

Accompanied by three vehicles and the beating of a drum, the group headed to St. Paul, their first stop on the AIM-sponsored walk that is expected to take them through Chicago, Cleveland and Pittsburgh before they wind up at the Lincoln Memorial in Washington in mid-November.

Peltier, 78, who supporters say has major medical problems, has been imprisoned for 45 years since his conviction for aiding and abetting the murder of two FBI agents on South Dakota's Pine Ridge Reservation in 1975.

In Washington, the walkers plan to make the case to federal officials that Peltier should have his sentence commuted, said Rachel Thunder, lead organizer of the walk. Peltier is being held at a high security federal penitentiary in Coleman, Fla.

"A piece of every single one of us is sitting in that cell with Leonard Peltier," Thunder told about 100 supporters, mostly Native Americans, at a rally Wednesday night in Cedar Avenue Field Park. "Until he is free, none of us is free."

"It's a sacred journey," said Mitch Walking Elk, who led a pipe ceremony attended by about 60 people Thursday morning, shortly before the walk began. "Everything we are doing is being done with prayer. This is a spiritual effort."

Among those on the walk was Linda Julik, 53, of Nett Lake, Minn. "My grandfather came to me in a dream," she said. "He was walking with me. I have to do this for my grandkids and future grandkids. [Peltier] is the longest-held political prisoner in the world."

Also on the walk were three AIM activists from Reno, Nev., and a group of five white people from Ipswich, Mass., who belong to the First Church, one of the oldest continuous meeting congregations in the country. "I felt something needed to be done," said Zeke Allman, 17. "You can't hear about this and ignore it."

"I think there's power when people stand together," said Sophie Watson, 26, a Dakota from Prior Lake, who was also on the walk.

Speakers at the rally and pipe ceremony included one of Peltier's daughters, Kathy, and her mother, Anne Begay, who thanked those going on the walk. "My heart goes out to each of you as you walk for Leonard," she said.

FBI agents Ronald Williams and Jack Coler were killed during a gunfight on the reservation on June 26, 1975. Two Native Americans arrested for the murders were acquitted by a federal jury in Cedar Rapids, Iowa. But Peltier, who was extradited from Canada, was found guilty in federal court in Fargo in 1977 and received two consecutive life sentences.

"I believe he is innocent," said Kevin Sharp, Peltier's Nashville-based attorney and a former federal judge, in an interview Wednesday. "The FBI and the U.S. Attorney's Office intimidated witnesses and the evidence tends to show he did not commit a crime."

Sharp said federal authorities had evidence that showed Peltier was not guilty but did not turn it over to the defense. He also said witnesses lied and prosecutors knew it.

Over the years, several presidents have been approached by Peltier's lawyers but refused to grant him clemency. Hundreds of FBI agents marched outside the White House in 2000 before President Bill Clinton rejected a clemency petition.

The FBI Agents Association, an organization of active and retired FBI agents that has led the opposition to Peltier's release, said Thursday that its position has not changed.

In a letter sent last December to U.S. Attorney General Merrick Garland, Brian O'Hare, the association's president, wrote: "Activists supporting Peltier use a mix of folklore, falsehoods and out-of-context statements that are designed to both exonerate Peltier in the eyes of the public, rather than show any true remorse regarding the murders of Agents Coler and Williams."

But pressure to release Peltier continues. On March 30, the United Nations Human Rights Council Working Group on Arbitrary Detention issued a lengthy statement calling for Peltier's immediate release. It raised questions about his trial, subsequent appeals and the pardon process, and said his continued incarceration is in violation of the Universal Declaration of Human Rights.

Lisa Bellanger, executive director and co-chair of the AIM grand governing council, is lead teacher in the American Indian education program in the St. Paul Public Schools. As the walkers set off Thursday, she carried a "Free Leonard Peltier" banner that she had taken on "The Longest Walk," a five-month march from San Francisco to Washington in 1978 to protest threats to tribal lands and water rights.

"We're going to raise the awareness, raise it to the sky," Bellanger said in an interview. She told supporters: "We're going to tell them in D.C.: 'The eyes of the world are on you.'"

Correction: A previous version of this story incorrectly characterized the membership of the FBI Agents Association. The organization includes both active and retired FBI agents.