The Vatican's ambassador to Burundi, who was shot dead on Monday, had cancelled his post-Christmas break to help bring peace, his sister has said.
The Vatican expressed dismay at the monsignor's death
Kathleen Vandenburg told Irish state broadcaster RTE that he had said: "Things are moving in the right direction, I could be useful."
Burundi's Catholic church has accused FNL rebels of killing Papal nuncio Michael Courtney, 58, from Ireland.
Meanwhile, six people have been killed in clashes between rebels and the army.
"I did ring him before Christmas, because he usually took a break after Christmas, and he sounded very optimistic," Mrs Vandenburg said.
The Vatican has expressed "deep sorrow" at the diplomat's death, describing him as "a faithful and generous servant of the
The FNL rebels refuse to take part in the peace process which has led to another rebel group joining the government and their fighters being integrated into the national army.
"The nuncio's appeals to the FNL to stop fighting and their reactions show, in my opinion, that we need not look for the culprits outside the FNL," said Burundi's most senior Catholic church official, Archbishop Ntamwana.
But the FNL has denied the
"We knew where he lived... We could have killed him if we wished. We strongly condemn those who killed him," rebel spokesman Pasteur Habimana said.
Thabo Mbeki, president of South Africa, which has played a key role in the peace process, also expressed his "outrage and condemnation at the senseless murder of Monsignor Michael Courtney".
Burundi President Domitien Ndayizeye, speaking from the hospital where Monsignor Courtney died, vowed to bring those responsible to justice.
He noted that the area where he was killed was a stronghold of the FNL rebels.
The Missionary Service News Agency (Misna) in Rome said that Monsignor Courtney had been travelling by car with three other people when their vehicle was sprayed with gunfire in Minago, Bururi province.
The other occupants escaped unharmed or with light injuries.
Mariano Benni of Misna told CNN that several sources had said the killing was an accident.
Monsignor Courtney had served as ambassador to Burundi since 2000.
Father Claudio Marano, a missionary in Burundi, told Vatican Radio that he had met all the various groups several times and the entire government.
"He was working a lot in favour of peace and reconciliation in Burundi," Father Marano said.
Some 300,000 people have been killed in Burundi's decade-old civil war, in which rebels of the majority Hutu
group are fighting to end dominance of the Tutsi minority.