Former president of Afghanistan Burhanuddin Rabbani and four of his body guards were killed in a suicide bomb explosion at his home Tuesday in Kabul.
The bomber reportedly smuggled the explosives in his turban, according to Kabul police.
The turban bomber entered Rabbani's house in the evening and blew himself up inside, said Mohammad Zahir, chief of criminal investigation for the Kabul police.
Fazel Karim Aimaq, former lawmaker from Kunduz province and a friend of Rabbani's, told reporters outside the former president's home that a man who arrived to meet Rabbani was not properly searched.
The visitor shook Rabbani's hand and bowed as a sign of respect, Aimaq said.
"Then his turban exploded," he said.
Rabbani served as president of Afghanistan from 1992 until 1996. He had led a group that was trying to find a political solution to a decade-long war with Taliban insurgents.
The Taliban had forced him to leave the country when they seized power.
The incident prompted current Afghan President Hamid Karzai to cut short his trip to the United Nations.
Analysts say Rabbani's death will impede efforts to keep in check the regional and ethnic rivalries that partly feed the insurgency.
President Barack Obama said the killing will not deter the U.S. and Afghanistan from helping that country's people live freely.
He said the former president's death is tragic because he was a man who cared deeply about Afghanistan.
Meanwhile, gunmen opened fire on a busload of Shiite Muslim pilgrims in Pakistan on Tuesday, killing 26 and injuring six others.
Witnesses said a group of at least eight men carrying rockets and guns forced the passengers off the bus. Some of the passengers tried to run away, but the gunmen opened fire.
The men were believed to be Sunni Muslims.
Pakistan is a majority Sunni Muslim nation. Although most Sunnis and Shiites in Pakistan live together peacefully, extremists on both sides target each other's leaders and activists.