Prosecutors did not say Spielman took part in the rape or murders, but alleged he knew the intention of officers in the group who committed the attacks.
He pleaded guilty on Monday to lesser charges including arson and obstruction of justice, wrongfully touching a corpse and drinking.
The trial in Fort Campbell, Kentucky, found that Spielman was among a group of soldiers who raped and killed al-Janabi, murdered her parents and sister and set fire to the family home.
Steven Green, the alleged head of the group, will face a civilian court because he was discharged from the army before the crime came to light.
Federal prosecutors have said they will be seeking the death penalty in Green’s case.
Earlier this year, Sergeant Paul Cortez and Specialist James Barker received life sentences after pleading guilty to raping al-Janabi.
Private Bryan Howard, who stood on guard during the attack, was sentenced to 27 months in jail for acting as an accessory as a lookout and obstructing justice.
Prosecutors rested their case against Spielman on Thursday to address Barker's story that Spielman acted as a lookout during the attack.
Barker had earlier testified that he had allowed investigators to draft sworn statements for him that implicated Spielman in the crime.
On Wednesday, Barker testified that parts of the document were untrue, including references to Spielman's role in the conspiracy to attack the family and his knowledge of plans to rape al-Janabi.
However, Cortez testified that Spielman stood guard as his fellow soldiers raped the girl a few feet away, and did nothing to stop the assault.
During their courts-martial, Barker and Cortez testified they took turns raping the girl while Green shot and killed her mother, father and younger sister.
Green shot the girl in the head after raping her, they said.
Her body was set on fire with kerosene to destroy any evidence, according to previous testimony.