The investigation was opened by the army's Criminal Investigation Command after a report by Afghan military prosecutors detailing the allegations was made public by a Washington-based human rights group called Crimes of War Project.
"It's under investigation," said Major Pamela Hart, an army spokeswoman.
The death of Jamal Nasir, an 18-year-old member of the Afghan army's III Corps, had not been previously investigated by the US military, she said.
"At first there were no records. They didn't have names of victims and alleged accusers," she said.
The Afghan investigation, excerpts of which were posted on the website of the Crimes of War Project, said Jamal Nasir and seven other Afghan soldiers were detained on 1 March 2003 by US forces who arrived suddenly at their post at a place called Sato Kandaw.
It quotes witnesses from among the seven surviving soldiers, including the recruit's older brother, as saying they were blindfolded, taken to a military camp at Gardez where they were interrogated for more than two weeks.
"My brother ... died in the camp due to the beatings and was martyred"
"There we were beaten up," Nasir Ahmad, Jamal's brother, said in a sworn statement. "It felt like they were hitting us with a cable or something made of rubber. This continued until the 17th or 18th."
"Eventually, my brother who was younger than me, died in the camp due to the beatings and was martyred at 1330 GMT," his statement said, according to a translation posted on the website.
The other seven were turned over to Afghan security forces and eventually released because there was no evidence to support the charges against them.
In a covering letter of 10 May 2003 submitting the military's investigative report to Afghanistan's attorney general, the Afghan armed forces attorney general Muhammad Tamkin said there was "a strong possibility that an individual by the name of Jamal, son of Ghazi, has been murdered as a result of torture by allied forces during his interrogation.
"Considering the above-mentioned incident and the 14th amendment of the Penal Code of Afghanistan, it is necessary for our legal system to investigate the torture of the seven individuals and the murder of Jamal, son of Ghazi, and other similar acts committed by foreign nationals," the letter said.