Allauddin Khan / AP

An Afghan villager, right, shows an empty canister allegedly used by US forces during Sunday's killing of civilians at a prayer ceremony for victims in Panjwai, Kandahar province, on March. 13, 2012.

Allauddin Khan / AP

Villagers listen to speeches during a prayer ceremony for victims of Sunday's killing, in Panjwai on March. 13, 2012.

Allauddin Khan / AP

Afghan security forces take up positions in a dried water canal after Taliban militants opened fire on a delegation of senior Afghan officials in Panjwai on March. 13, 2012.

Suspected insurgents opened fire on Tuesday on senior Afghan investigators of the massacre of 16 civilians by a lone U.S. soldier, Afghan officials said, just hours after the Taliban threatened to behead American troops to avenge the killings.

The gunmen shot from long range at two of President Hamid Karzai's brothers, Shah Wali Karzai and Abdul Qayum Karzai, and security officials at the site of the massacre in Kandahar's Panjwai district.

Karzai's brothers were unharmed in the brief battle, which began during meetings with local people at a mosque near Najiban and Alekozai villages, but a soldier was killed and a civilian wounded. 

In Washington, President Obama said he viewed the killing of 16 Afghan civilians as seriously as if those killed had been Americans.

"The U.S. takes this as seriously as if it were our own citizens and our own children who were murdered," Obama said at the White House.

-- Msnbc.com and news services contributed to this post

Slideshow: Afghanistan: Nation at a crossroads

Jangir / AFP - Getty Images

More than ten years after the beginning of the war, Afghanistan faces external pressure to reform as well as ongoing internal conflicts.

Launch slideshow

Allauddin Khan / AP

An elderly Afghan man sits next to a covered body, who was allegedly killed by a U.S. service member, in a minibus in Panjwai, Kandahar province south of Kabul, Afghanistan, March 11. Villagers showed an Associated Press photographer 15 bodies, including women and children, and alleged they were killed by the American.

A U.S. service member killed at least 15 members of two Afghan families as well as a 16th person before turning himself in, witnesses and officials said Sunday. Nine of the dead were children, and three were women, Afghan President Hamid Karzai said in a statement.

The soldier, who has yet to be identified, reportedly left his base in the early hours Sunday and went to two villages just a few hundred yards away. He then opened fire on Afghan civilians sleeping in ther homes, Minister of Border and Tribal Affairs Asadullah Khalid told Reuters. The service member entered three homes in the villages in Kandahar province, he said.

-- Reported by msnbc.com staff and news services

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