With hundreds of volunteers preparing an elaborate burial site, a youth choir rehearsing songs of mourning and throngs of southern Sudanese arriving on foot from their villages, this Nile River city prepared for the funeral Saturday of John Garang, a rebel icon turned peacemaker and statesman.

But Ugandan President Yoweri Museveni injected a disturbing note into the preparations Friday when he told a crowd of mourners in a southern Sudan town that the cause of the helicopter crash that killed Garang was not clear. His comments were in sharp contrast to statements by Garang's family and associates, who have said bad weather was the culprit.

"Some people say accident. It may be an accident, it may be something else," Museveni said, according to radio and news agency reports. He said the helicopter was "very well equipped . . . the one I am flying all the time. I am not ruling anything out. Either the pilot panicked, or there was some side wind, or the instruments failed, or there was an external factor."

Garang was flying to his base village, New Site, from a visit to Museveni's ranch when the Ugandan Mi-72 military helicopter crashed into a rugged mountain range near New Point, Sudan. Museveni spoke in Yei, one of the towns where Garang's body was being taken before the funeral here.

Many southern Sudanese have said they believe Garang's death was a political killing, and some have blamed the Khartoum government, against which his rebel movement fought for years until a peace agreement was reached in January. News of his death unleashed several days of rioting and looting in which officials said at least 130 people died.

Museveni has long had hostile relations with Khartoum. He has openly accused the Arab-led government of trying to overthrow him because it backs a rebel group in northern Uganda. On Sunday, investigators from Sudan, Kenya and Uganda are expected to begin an official investigation into the cause of the crash, in which all 15 people on board were killed. The United States and Britain have also offered to send experts. Officials said the helicopter's voice recorder has been found.

Civic leaders and officials from Garang's Sudan People's Liberation Movement cautioned that Museveni's comments were vague and should not be taken literally because of the risk of intensifying public emotions and prompting further violence.

"We have to give the investigation a chance," said Gen. Pieng Deng, an official of the rebel group who was setting up security for the funeral. He said everyone should remain calm, adding, "Right now, we do believe it was an accident."

Garang, a burly, scholarly guerrilla leader, was 60 when he died. On July 9, he was made Sudan's first vice president under a peace deal that ended 21 years of war between the Arab north and the African south.

The peace agreement, the product of protracted negotiations, was closely identified with Garang's strong and respected personality. His death left the country angry and anxiously wondering whether the hard-won peace would crumble.

But in Juba on Friday, the mood was calm and focused on preparations for a grand state funeral. More than 300,000 mourners were expected to gather, including President Omar Hassan Bashir, whom Garang once vowed to topple.

"Dr. Garang was like a father to us," said Peter Bii, 18, who leaned over an organ in All Saints Anglican Church, practicing songs he had composed for the ceremony. Bii said he had recently returned from refuge in Uganda and learned that his own father, a rebel fighter, had died. "Oh, Dr. Garang, set us free," he sang. "We will always remember you for a thousand and a million years."

In a grassy field a mile away, more than 50 volunteers sweated heavily in the summer heat as they dug the grave and laid tile for a star-shaped mausoleum above it. Joseph Rombe, 31, said he had joined in "to honor the power and pride of the man."

Garang was both a Christian and a proud Dinka tribesman; the funeral is being planned to incorporate both traditions.

The preparations also included heavy security arrangements. Troops from President Bashir's elite guard arrived in two planes, fanned out in pickup trucks and lined the road to the church. Heavily armed soldiers guarded the church as a children's choir rehearsed, and watched as a military band from Garang's rebel forces practiced.

At the airport, soldiers crouched over rocket launchers, and several hundred Arab traders were evacuated in the troop planes. Many had waited four days to flee after their shops were looted and burned by angry southern Sudanese.

Volunteers handed out multicolored New Sudan flags. Church and relief groups brought cooking pots, cases of bottled water, tents and foam mattresses to handle the crush of visitors in Juba, which has no paved roads, no running water and just a handful of hotel rooms.

Garang's son Chol, 25, a fine arts student in Britain, dug the first scoop of earth for his father's grave Thursday night. Chol, wearing dreadlocks and jeans, said he had attended the July 9 swearing-in celebration in Khartoum and then accompanied his father to New Site.

"He was in a very good mood," the son said. "I had never seen him in such good spirits."

Sometimes, Chol said, "I think I will wake up and see him, and I find he's gone and people are crying." The artist said that he had been inspired by the open land and sky of his father's native region and that Garang had approved of his vocation.

"He died a free man," Chol said.

A crying Nyibol Machak is helped by daughters Akuia Maiyor, left, and Mary Ayor Maiyor after visiting the grave site of former rebel leader John Garang.President Yoweri Museveni of Uganda said cause of crash was not clear.Men work on Garang's grave, to be enclosed in a star-shaped mausoleum. One volunteer said he joined in "to honor the power and pride of the man."