A US soldier has been charged with killing 17 Afghan civilians, including women and children, and attempted murder of six others while posted in Kandahar on March 11.
The soldier, who on Friday was officially identified as Army Staff Sgt Robert Bales, faces death penalty.
"It's the first step in this process," Pentagon spokesman Navy Capt John Kirby said during an off camera news conference.
Criminal charges to Bales have been filed under the Uniform Code of Military Justice (UCMJ).
Bales is assigned to 2d Battalion, 3d Infantry Regiment, headquartered at Joint Base Lewis-McChord in the Washington State.
He is currently in pre-trial confinement at Fort Leavenworth, Kansas.
The next procedural step in the military justice process is for the Special Court-Martial Convening Authority at Joint Base Lewis-McChord to decide whether to direct a probe of the charges under UCMJ Article 32.
An Article 32 investigating officer submits to the command a written report with non-binding recommendations concerning the sufficiency of the charges and evidence.
The report aids the command in determining an appropriate disposition of the charges.
The charges may not be referred to a general court-martial absent a prior Article 32 investigation.
At the minimum, Bales faces a sentencing to life in prison with the possibility of parole if he's convicted on even one of the 17 murder charges.
He could face the death penalty at the maximum.
Kirby said the investigation is still ongoing and it is possible that more charges could come until the investigation is closed.
In an email to the CNN, the Taliban vowed "strong revenge" for the attacks and claimed justice won't be served in US courts, which they said "are not reliable".
"We don't believe in these (American) courts and reject the decision," the Taliban said.
"We will take practical revenge on every single American soldier," it said.
Bales' lawyer, John Henry Browne, said the case is going to be "extremely difficult" for the prosecution.
"They have no murder scene, no forensics. I am going to make them prove every claim," he added.