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LONDON - NATO will investigate whether a grenade thrown by American military forces killed a British aid worker during a rescue attempt in Afghanistan last week, an alliance spokesman said Monday.

Linda Norgrove, 36, was killed Friday in the raid by U.S. forces in Afghanistan's eastern Kunar province, after she and three colleagues were kidnapped two weeks earlier. NATO initially said Norgrove died when captors detonated a bomb as NATO forces attempted to free her.

However, British Prime Minister David Cameron said Monday that Norgrove was possibly killed by a grenade lobbed by a member of the U.S. special forces rescue team.

Cameron said he informed Norgrove's family of the "deeply distressing development," and defended the decision to attempt the risky rescue mission.

"We were clear that Linda's life was in grave danger and the operation offered the best chance of saving her life," Cameron told reporters in London.

Lt. Col. John Dorrian, a spokesman at NATO headquarters in the Afghan capital Kabul, said Monday the rescue mission leader saw surveillance footage of the raid, talked with members of the rescue team, and decided "it was not conclusive what the cause of her death was."

The rescue mission leader spoke with the commander of U.S. and NATO forces in Afghanistan, Gen. David Petraeus, who requested the investigation, Dorrian said. The probe will be led by U.S. Army Maj. Gen. Joseph L. Votel of U.S. Special Operations Command.

"It is our solemn responsibility to understand the circumstances that led to her death," said Petraeus. "We will provide every measure of support to the investigation, and will work closely with the British government to fully resolve this matter."

NATO Secretary-General Anders Fogh Rasmussen said in Brussels the alliance will wait for the final outcome of the investigation before commenting.

"But whatever happened, I would like to stress that those who are responsible (for her death) of course are the captors," Fogh Rasmussen said.

Norgrove, who worked for U.S.-funded Development Alternatives Inc., was abducted in an ambush on Sept. 26 along with three Afghan colleagues who were later released. Six kidnappers also died in the rescue attempt.

NATO was also investigating Monday the deaths of two civilians in southern Afghanistan a day earlier. Initial reports indicated they were killed in a NATO airstrike.

A joint force was attacked with small-arms fire in Kandahar on Sunday, NATO said. Troops called in an airstrike and followed up by firing mortar rounds in Zhari district.

"Two civilians may have been accidentally killed," said NATO, adding a child was also wounded. One insurgent died, it said.

An explosive device planted by insurgents killed three people and seriously wounded a child in southeastern Zabul province Monday, NATO said. An Afghan civilian also died in a roadside bombing in eastern Khost province.

The nine-year war has inflicted a mounting toll on Afghan civilians. A UN report said more than 1,200 Afghans died and nearly 2,000 were wounded between January and June this year.

In other violence, a roadside bomb killed a NATO service member in the south, the alliance said, without giving a nationality or exact location.

Monday's death brought to 27 the number of NATO forces killed this month. At least 2,015 NATO service members have died since the U.S.-led invasion of Afghanistan on Oct. 7, 2001, according to an Associated Press count.

In the east Monday, Taliban fighters ambushed a supply convoy guarded by Afghan military contractors as it traveled through Ghazni province on its way to Kandahar, said provincial chief of police Zarawar Zahid. An hourlong gunbattle killed eight insurgents and wounded two Afghan security contractors in Qarabagh district.

Six militants died in operations by Afghan forces Sunday in southern Helmand province's Marjah and Greshk districts, the Defense Ministry said in a statement Monday.

President Hamid Karzai confirmed his government has been in informal talks with the Taliban on securing peace in war-weary Afghanistan "for quite some time" — the latest in a series of high-level acknowledgments of contacts with the insurgent group.

Unofficial discussions have been held with Taliban representatives over an extended period, Karzai told CNN's "Larry King Live" in an interview to be broadcast Monday.

The Afghan government says it hopes to make talks more structured with a "peace council" that will aim for formal talks with insurgent groups. On Sunday, former President Burhanuddin Rabbani was named chief of the council.

Richard Holbrooke, the Obama administration's special representative to Afghanistan and Pakistan, said militants must renounce al Qaeda, lay down their arms, and respect the country's constitution -- particularly provisions protecting minorities and women.

Holbrooke told journalists and politicians in Berlin on Monday the talks did not involve the United States, and warned media were exaggerating the importance of the discussions.

"The reports greatly exceed the reality," Holbrooke said. "There's no question that the Taliban are under tremendous military pressure, and that's one of the reasons you've seen so much more recent comment about reconciliation."

NATO will meet in November in Lisbon, Portugal, to discuss the situation in Afghanistan. The alliance's chief said Afghan forces assuming a greater role in the war would be a major topic.

Fogh Rassmussen also addressed the readiness of Afghan forces, saying he expected an announcement in Lisbon that the transition to Afghan-led security would begin in parts of the country by January at the earliest, July the latest.