On Wednesday, Howard told a judge at a hearing in Kentucky, US, that he was left behind at a checkpoint while four other soldiers went to rape the girl, and that he had overheard the four planning the attack.
"I was slowly starting to believe what they had done, that they had committed the crimes, the rape and the murder," Howard said.
He said he only realised that someone had been killed after the soldiers returned about 10 minutes later, four of them in a "hectic state and hyper", and with blood on one of the soldiers' uniforms.
Two of the soldiers who previously pleaded guilty were sentenced to between 90 and 100 years in prison.
Captain Megan Shaw, Howard's lawyer, pleaded for leniency in sentencing and asked that he be allowed to return to his unit because he "was a good kid that deserves a second chance".
But Captain Alexander Pickands, a military prosecutor, said Howard "knew right from wrong".
"If I could go back, I would not have let it happen in the first place, and I definitely would have told someone"
Bryan Howard, third US soldier convicted in Iraqi rape case
"He knew that when his squad members returned bloodied, that something had gone horribly wrong. Knowing that, he chose to shield them," Pickands said.
During the sentencing hearing, Howard expressed regret for not stopping the killings and for covering up the truth, and apologised to the military, his family and to the girl and her family.
"Finally I want to apologise to the Iraqi people and the victims,'' he said in a statement to the court.
"If I could go back, I would not have let it happen in the first place, and I definitely would have told someone."