WASHINGTON (CNN) -- A formal investigation into an attack on a U.S. Army unit by about 200 Taliban insurgents will examine whether the Army had intelligence about a possible assault and whether the troops had access to it.
Taliban militants take up a defensive position at a undisclosed in Ghazni province, Afghanistan, on Saturday.
The fact-finding mission was launched Thursday, several military officials said, after nine American soldiers were killed in Sunday's assault in Afghanistan.
When the attack occurred, the U.S. and Afghan soldiers were scouting for a location in the remote area to set up a coalition observation point. The Taliban never breached the main coalition base near the village of Wanat in Kunar province, which borders Pakistan.
It was the deadliest attack on U.S. troops in Afghanistan in three years, underscoring how the conflict is escalating.
Since May, the deaths of U.S. and allied troops have far outpaced the toll in Iraq. On Thursday, the toll in Afghanistan was 21, compared with six in Iraq.
NATO's International Security Assistance Force did not provide the nationality of a soldier who died in Afghanistan on Thursday. See casualty figures for Afghanistan, Iraq »
U.S. military officials are searching for ways to send more troops to Afghanistan in response to urgent requests from commanders there, given the increase in violence. Watch how U.S. trying to boost Afghan force »
A senior U.S. Army official says "all options are on the table," including the possibility of diverting a combat brigade of up to 3,000 troops to Afghanistan later this year. Those troops are now scheduled to go to Iraq.
The commanders are asking for troops as soon as possible. Several officials say it's most likely that the fastest option would be to send a small number of "enabler" troops such as security forces, helicopter units and surveillance aircraft. There is a Marine Corps unit in the region on standby for emergencies that could be sent, officials said.
In Afghanistan on Thursday, local security forces and coalition soldiers in western Afghanistan killed several insurgents in what the NATO command called a "successful operation against high-priority Taliban targets."
The operation took place in the Shindand district of Herat province. Two Taliban leaders, Haji Dawlat Khan and Haji Nasrullah Khan, and a "significant number of other insurgents" were killed, according to a statement from NATO's International Security Assistance Force.
There was no evidence of civilian casualties or accidental damage in the operation, in which a "number of men were discovered handcuffed and imprisoned in appalling conditions in one of the insurgent compounds."
One of the toughest fronts in the war has been the southern province of Helmand.
In Helmand province in southern Afghanistan, the British Defense Ministry said, British troops killed a senior Taliban leader.
Mullah Bismullah Akhund was killed Saturday in the Now Zad district of Helmand, long a Taliban bastion.
The Defense Ministry, in a statement Wednesday, called Bismullah "a senior key facilitator and logistician responsible for the northern Helmand region." The ministry says his death will disrupt the Taliban's leadership structure and hamper the group's ability to conduct attacks.
"He is believed to have commanded numerous fighters and was identified by Task Force Helmand as a key player in the insurgency, and criminality, before the strike," according to the International Security Assistance Force.
British troops, who are part of the assistance force, announced the killing Thursday. Saturday's operation occurred 15 days after British troops killed another senior Taliban militant, Sadiqullah, in an Apache missile strike.
"Bismullah was closely associated with local Taliban leader Mullah Rahim, whose brother was also killed during this operation," the International Security Assistance Force said.
The Defense Ministry said that "combined with the elimination of Sadiqullah, this is the most significant blow struck against the Taliban logistics and facilitation chain in northern Helmand this year."
The U.S.-led coalition said it also is investigating an airstrike in western Afghanistan's Farah province. Launched after a coalition convoy was attacked Tuesday, it struck a house and killed eight civilians.
CNN's Barbara Starr and Jamie McIntyre contributed to this report.