As they mourned the slain Afghan leader who had been tasked with reconciling with the Taliban, close friends and associates of Burhanuddin Rabbani expressed anger and hopelessness Wednesday about peace talks they say were doomed from the start.

“We cannot continue to have wishful thinking,” former presidential candidate Abdullah Abdullah, who was close to Rabbani, said as he drove to the former president’s house to pay his last respects. “We must deal with them forcefully.”

Abdullah called President Hamid Karzai’s attempts to get Taliban leaders to lay down their arms and join the government naive.

“These are the people President Karzai calls ‘dear brothers,’ ” Abdullah said of Karzai’s conciliatory approach to the militants. “The ones who have killed thousands of people.”

Afghan officials said they were all but certain that the Taliban’s leadership or a faction of the group carried out Tuesday’s assassination, the latest high-profile killing in Afghanistan this year — and arguably the one most detrimental to efforts to end the 10-year-old war.

As of Wednesday night, no group had asserted responsibility for the attack. Taliban spokesmen on Wednesday were uncharacteristically hard to reach. A Twitter account that purports to be linked to the Taliban’s media operation posted messages Wednesday saying the group is not ready to weigh in on the killing.

“Since we have not yet completed our investigation into the matter, therefore our stance is that of not elaborating on the issue any further,” the message said.

Abdullah and other Afghan officials said the man who detonated explosives hidden inside a turban as he greeted Rabbani used the name Esmatullah.

Kabul was awash with rumors and speculation about the motive.

Some Afghans said it sent an unmistakable signal that insurgent leaders saw no benefit
to negotiating with Karzai’s U.S.-backed government. Others saw a narrower motive, saying that whoever carried out the attack perhaps felt that Rabbani, an ethnic Tajik who led the fight against the Taliban during the civil war in the 1990s, was the wrong man to broker a deal with the Pashtun militant group.

Senior figures of the High Peace Council that Rabbani led hosted the assailant for several days at a guesthouse in Kabul used by the peacemaking body, Afghan officials said.

An account provided by Amanullah Paiman, a former aide to Rabbani, described a sense of optimism about the envoy’s visit.

He said Mohammad Masoom Stanekzai, a senior member of the council, had been told by the envoy that he had an important message for Rabanni from the leadership of the extremist group’s leadership council, known as the Quetta Shura. In several phone calls, Stanekzai pressed Rabbani, who was traveling in Iran and Dubai, to head back to Kabul, said Ismail Qasimayar, a member of the peace council.

Afghans close to Rabbani said it was hard to believe that the assailants could have carried out the attack without help from members of Rabbani’s inner circle.

“I am sure that without inside and outside help this incident would not have happened,” Paiman said. “This man was kept in the guesthouse for a week, and nobody paid attention to his movement, turban and acts.”

On Wednesday morning, police closed off a prominent road in downtown Kabul that runs near Rabbani’s street to all but the throng of warlords and politicians who traveled to his residence in sport-utility vehicles with large posses of bodyguards to offer condolences to relatives.

As edgy guards armed with AK-47 assault rifles scanned the crowd, elderly bearded men embraced solemnly. Posters of the slain former president, who led the country from 1992 to 1996, were plastered around the neighborhood.

Ahmad Zaeem, a former assistant to Rabbani, looked ashen as he spoke outside Rabbani’s house.

“There is no place left for people to negotiate,” he said. “They attacked the foundation of peace in the country. There is no room for negotiation.”

While stunned by the audacity of the attack, some Afghans said they deemed the death inconsequential.

“Since the High Peace Council was established, there has been no achievement,” said Bizmillah, an army officer who provided only his first name. “They’re in it for the money.”

Ahmad Wali Massoud, a close friend and political ally of Rabbani’s, said the former president had dim hopes that Karzai’s peace plan could work.

“He made his own effort,” said Massoud, a brother of the renowned commander Ahmed Shah Massoud, who helped drive the Soviets out of Afghanistan and was killed a decade ago in a strikingly similar attack. “But I’m sure he did not believe this could work.”

Massoud said the prospect of a negotiated settlement appears dead.

“I don’t think the enemy is that strong,” said Massoud, who served as ambassador in London during Karzai’s first term in office. “The problem is that the government under Karzai is so weak, so fragile, so corrupt that the terrorists now have the upper hand.”

Special correspondents Sayed Salahuddin and Javed Hamdard contributed to this report.