The cause of the helicopter crash which killed veteran southern Sudanese leader John Garang is "not clear", Ugandan President Yoweri Museveni has said.
Garang: rebel leader for 21 years, vice-president for three weeks
All previous official comments have insisted that the crash was an accident. Mr Garang was travelling in Mr Museveni's helicopter when he died.
Mr Garang's death sparked three days of riots in Sudan, in which at least 130 people were killed.
Mr Museveni was speaking to thousands of mourners in southern Sudan.
"Some people say accident, it may be an accident, it may be something else," he said in Yei, one of the southern towns where Mr Garang's body is being taken before the funeral on Saturday.
"The [helicopter] was very well equipped, this was my [helicopter] the one I am flying all the time, I am not ruling anything out," Mr Museveni said.
The official investigation into the causes of the crash has not yet started but the helicopter's voice recorder has been found.
The BBC's Caroline Karobia said hundreds of mourners wept when they saw Mr Garang's coffin in Yei.
Mr Garang died three weeks after being named vice-president as part of a deal to end 21 years of civil war.
Hundreds of people - many in tears - have been turning out to see Mr Garang's coffin when the plane arrives in each town.
In Juba, where the funeral is due to take place, thousands of volunteers are helping to build a mausoleum to house Mr Garang's remains.
Mr Garang's son, Chol, started the work and a cow was slaughtered, in keeping with the traditions of the Dinka community.
A huge crowd is expected for the funeral, including the presidents of Sudan, Uganda and South Africa.
Saturday has been declared a national holiday.
Tensions remain high in Juba after communal clashes, sparked by Mr Garang's death.
Northern Arab traders were attacked and their goods looted by angry southerners, leaving at least 19 dead. Food is reportedly running short.
Mr Garang's widow and his children are accompanying the body from New Site, near where the helicopter crashed.
"We have to keep his [Garang's] promise. We have to implement the peace agreement," Riak Machar, one of Mr Garang's former colleagues told the crowd in Kurmuk on Thursday.
After Yei, the one-time base of Mr Garang's rebel SPLA, the funeral procession goes to Bor, where the rebellion started in 1983 and his birthplace of Panyagol near Bor, before reaching Juba.
The capital, Khartoum, is reported to be calm, with no more of the clashes between southerners and Arabs sparked by Mr Garang's death.
More than 130 people were killed in three days of violence.
Northern-owned shops were burned out in Juba
The police say that more than 2,000 people have been arrested in Khartoum for their part in the violence.
Some have already been sentenced to six months in jail.
An overnight curfew has been imposed and heavily-armed police and soldiers are on patrol.
The US has expressed deep concern over the situation.
Leaders of both sides earlier urged calm, agreeing to set up a joint inquiry into what caused the crash of Mr Garang's helicopter.
But correspondents say there is a real danger that a much-vaunted peace deal ending 21 years of civil war could be shattered by the sudden violence.