Gunmen killed a senior leader of one of the most feared militant groups fighting U.S. troops in Afghanistan when he stopped at a bakery on the outskirts of Pakistan’s capital, members of the militant organization and an eyewitness said Monday.

Nasiruddin Haqqani’s death Sunday night is one of the biggest blows to the Haqqani militant network since the Afghan war started, but the commander’s presence in Islamabad and questions over who killed him could spark fresh tension between Pakistan and the United States.

U.S. officials have accused Pakistan’s intelligence agency of supporting the al-Qaeda-linked Haqqani network to counter the influence of archenemy India in Afghanistan — an allegation denied by Islamabad.

No one has claimed responsibility for killing Haqqani in an area of Islamabad only a couple of miles from the U.S. Embassy, but some Pakistanis suspect that Americans were behind it. That’s a contentious allegation in a country angered by U.S. drone attacks targeting militants in Pakistan’s tribal region that many see as a violation of state sovereignty. The U.S. Embassy said it was looking into the matter.

Haqqani was a key financier and emissary for the family business. His brother, Sirajuddin Haqqani, leads the group, which was founded by their father, Jalaluddin Haqqani, who achieved fame fighting the Soviets in Afghanistan in the 1980s.

The U.S. Treasury imposed sanctions on Nasiruddin Haqqani in 2010 when it added him to its list of specially designated global terrorists. The Treasury said he has traveled to Saudi Arabia and the United Arab Emirates to raise money for the Haqqani network, al-Qaeda and the Taliban.

The gunmen who killed Haqqani as he got out of his vehicle at a bakery in a residential area called Bara Kahu were riding a motorcycle, witness Tanveer Ahmed said. He said he was nearby when the shooting took place but only learned the dead man’s name later. The attack left blood on the pavement and bullet holes in the bakery’s tiled wall.

Ahmed said that when he heard the shooting he first thought it was celebratory fire, which is common at weddings and other events in Pakistan.

Haqqani’s body was taken to the town of Miran Shah in the North Waziristan tribal area, according to Ahsanullah Ahsan, a Pakistani Taliban commander. Haqqani was buried Monday, another Pakistani militant said.

— Associated Press