"The (helicopter) was very well equipped, this was my (helicopter) the one I am flying all the time, I am not ruling anything out," he said, noting that an international panel of experts had been appointed to look into the crash.

"Either the pilot panicked, either there was some side wind or the instruments failed or there was an external factor," Museveni told southern Sudanese mourners in the town of Yei where Garang’s body was brought ahead of his Saturday funeral in Juba.

"The inquiry will look at all possibilities," he said. His comments were met with stony silence from the crowd, which had earlier greeted the arrival of Garang’s coffin with wailing, ululation and prayer.

Garang’s successor as chief of the Sudan People’s Liberation Movement/Army (SPLM/A), Salva Kiir, declined to comment on the specifics of Museveni’s remarks but said the group was eager to hear the results of the probe.

"All options are open and cannot be ruled out," Kiir told reporters after telling the mourners to reject the violence that has engulfed parts of Sudan since Garang’s death and urging them to hold to their ex-leader’s vision for peace.

"Let us follow the footsteps of our leader," he told the crowd.

"This is not the time for rioting," Kiir said, blaming unspecified opponents of peace for wanting "to provoke a situation that would lead us back to war."

SPLM/A spokesman Pagan Amun told AFP in Khartoum: "We don’t have anything to suggest it was caused by sabotage.

"We do not want to prejudice the investigation simply because we do not know. People should not rush to conclusions."

Garang and 13 others died when Museveni’s presidential Mi-172 helicopter went down in the mountains of southern Sudan, sparking days of violence in Khartoum and the south that saw 130 were killed and hundreds wounded.

His death and the subsequent rioting also raised fears of the unraveling of the landmark January peace deal he signed with Khartoum that ended Sudan’s 21-year north-south civil war.

However, relative calm returned to the streets of Khartoum on Friday as shop-owners re-opened for business amid a noticeably lower security presence.

Many southerners believe that Garang was somehow assassinated and have refused to believe the initial insistence of Ugandan, Sudanese and other officials that the helicopter went down because of poor weather.

Museveni did not elaborate on what he meant by "external factor" but there has been widespread speculation in southern Sudan that the chopper may have been somehow sabotaged or shot down.

However, until Friday officials in Khartoum and Kampala, members of the SPLM/A leadership and Garang’s widow, Rebecca, had all dismissed such possibilities and maintained the crash was accidental.

In a bid to quell the speculation and restore calm, Sudanese President Omar el-Beshir announced on Wednesday that a formal inquiry into the incident would be conducted.

Museveni said Friday that aviation experts from Uganda, Kenya, the United States, Britain and Russia would join the investigation.

His comments came as thousands of southern Sudanese volunteers descended on a former military training ground in Juba to help prepare Garang’s burial site ahead of his funeral on Saturday that is expected to draw some 500,000 people.

Weeping women, stoic men and curious children converged on the site near the state parliament where frantic construction was underway on a mausoleum to hold Garang’s remains.

A small group of UN peacekeepers, deployed in Juba after Garang signed the pact with the government in Khartoum, assisted the volunteers with heavy equipment as several women fainted in the extreme heat.

In accordance with Dinka tribal tradition, one of Garang’s sons, Chol, a 25-year-old fine arts student in Britain, dug up the first chunk of earth where his father will be laid to rest.

"It is a shock, it seems like a dream," he told reporters of his father’s sudden death, news of which sparked several days of deadly clashes between northern and southern Sudanese in Juba, the southern town of Malakal and Khartoum.