Burhanuddin Rabbani, a former president of Afghanistan, has been killed in a suicide attack in the country's capital, Kabul.
Rabbani, who served as president in the 1990s, was recently made the head of the High Peace Council, tasked by Hamid Karzai, the current president, to reach out to the Taliban.
Mohammad Zaher, Kabul's criminal investigations chief, said two men "negotiating with Rabbani on behalf of the Taliban" arrived at his house on Tuesday, one with explosives hidden in his turban.
"He approached Rabbani and detonated his explosives. Rabbani was martyred and four others including Masoom Stanekzai [his deputy] were injured".
Fazel Karim Aymaq, a member of the High Peace Council, said the men claimed to have come with "special messages" from the Taliban and were thought to be "very trusted."
When Rabbani appeared, the man shook the former president's hand and bowed in a sign of respect, Aymaq said. "Then his turban exploded.''
The blast broke windows in Rabbani's home and shook nearby houses.
Initial reports said four bodyguards had been killed but Zaher said this was incorrect.
The latest in a series of targeted killings, Rabbani's is the most high-profile political killing since 2001.
He was president of the Afghan government that preceded the Taliban, a period of civil war that saw thousands of people killed.
After he was driven away from Kabul by the Taliban in 1996, he became the nominal head of the Northern Alliance, which swept to power in the capital after the Taliban's fall in 2001.
President Karzai, who cut short a trip to the United Nations in New York after hearing of the attack, called on Afghans to remain unified in the face of Rabbani's "martyrdom". An emergency cabinet meeting was called for on Wednesday.
The UN Secretary-General Ban Ki-moon also condemned the killing and underscored the UN's commitment to "supporting Afghanistan and its people attaining peace and stability and to working in close co-operation with them," his spokesman said.
Asif Ali Zardari, Pakistan's president, and Yousuf Raza Gilani, the prime minister, also condemned the attack.
Amrullah Saleh, a former intelligence chief who fought against the Taliban under Rabbani, told Al Jazeera the attacks showed the government's failure in protecting high-profile figures.
"These attacks tell us that the policy of appeasement and deal making with the Taliban and Pakistan is not going to lead to peace.
"By adapting a vague policy of so called reconciliation, [the government] has created confusion in our society
and weakened the government to the extent that they can't even protect high-profiled leaders in the capital."
Rabbani's death could also unleash the resentment building up among some senior Northern Alliance members, who have criticised Karzai for his peace efforts with the Taliban.
"If Karzai wants to keep Afghanistan united, he has to launch massive massive investigations and bring the culprits to justice."