A post-mortem examination showed Mr Mousa had at least 93 injuries
A former British soldier has admitted for the first time that he saw two of his colleagues kicking and hitting an Iraqi prisoner shortly before he died.
Garry Reader told a public inquiry how, then a private, he had tried in vain to resuscitate Baha Mousa in 2003.
He said he had not told the truth previously, but did believe Cpl Donald Payne and Pte Aaron Cooper had caused Mr Mousa's death that September.
Mr Reader said he had been afraid speaking out would damage his career.
Mr Mousa and nine other civilians were arrested at a hotel in Basra in 2003 and taken into UK military custody.
The father-of-two died the following day, having suffered 93 separate injuries, including fractured ribs and a broken nose.
Cpl Payne became the first member of the British armed forces to be convicted of a war crime when he pleaded guilty at a court martial in September 2006 to inhumanely treating civilians.
He was dismissed from the Army and sentenced to one year in a civilian jail.
Six other soldiers who faced the court martial were all cleared on all counts in March 2007.
'Not a threat'
Mr Reader told the inquiry in London that he had seen Mr Mousa the day after he was arrested.
He said he was handcuffed and looked "dazed", and was not wearing a sandbag hood on his head like other detainees.
He said he then saw Cpl Payne and Pte Cooper grab Mr Mousa and force him into a room.
"I don't believe he was a threat. I do not even believe he was trying to escape, I just think he was injured and wanted to get help," Mr Reader said.
"I saw them struggling with him. One of them - I cannot remember which one - was trying to get the sandbag on his head. Baha Mousa was struggling and he seemed to be trying to break free.
"I saw Payne and Cooper kicking and hitting Baha Mousa trying to get him in the room."
Mr Reader said Mr Mousa was "screaming" and at the sight of the treatment he had chosen to leave the room.
"I just walked out of the room... because I wanted nothing to do with that. This was not what I had signed up for," he said.
Mr Reader then told of returning to the room 10 minutes later and finding Mr Mousa slumped on the floor and unresponsive.
He said he tried to give the Iraqi artificial resuscitation until a medic arrived and took him away on a stretcher.
'Banged his head'
Mr Reader, who left the Army in July 2007, told the inquiry that six years after the event he felt "able to speak more candidly".
"At the court martial I was still a serving soldier with a family. I wanted to protect my family. I didn't want any repercussions, that is why I didn't speak up."
He added: "I believe that Corporal Payne and Private Cooper caused the death of Baha Mousa as I have described above."
The BBC's Caroline Hawley said another former soldier Gareth Aspinall had also given evidence to the inquiry on Monday.
He told of seeing Mr Mousa's body being carried on a stretcher and of hearing Cpl Payne say, 'If anyone asks, he banged his head', our correspondent said.
The inquiry continues.