The revelations follow the release on Tuesday of internal US navy documents as a result of a court order.
Obtained by the American Civil Liberties Union (ACLU), the documents show that marines were punished in some instances while other cases were closed after investigations concluded the allegations could not be substantiated.
The ACLU said the documents showed that abuse and even torture of detainees by marines in Iraq was widespread.
A spreadsheet on detainee abuse cases investigated by the Naval Criminal Investigative Services (NCIS) said the alleged mock execution was one of several incidents involving four marine suspects in al-Diwania between 1 June and 6 July 2003.
They were alleged to have "ordered four juvenile looters to kneel beside two shallow fighting holes and pistol was discharged to conduct a mock execution", the document said.
Two suspects were found guilty of dereliction of duty and sentenced to 30 days of hard labour, while another was reduced in rank, forfeited two-thirds pay for a month and placed under unspecified restriction for 14 days after being found guilty of detainee abuse.
Charges against the fourth were withdrawn, according to the document.
Another entry shows that five marines were alleged to have taken part in shocking a detainee with an electric transformer at a holding area at al-Mamudia in April 2004.
"This kind of widespread abuse could not have taken place without a leadership failure of the highest order"
ACLU executive director
An unidentified witness reported that "the detainee danced as he was shocked".
A general court martial in May 2004 found one marine guilty of "assault, cruelty and mistreatment, dereliction of duty and conspiracy to assault a detainee", the document said. He was sentenced to a year in prison.
Another marine was sentenced to eight months in prison in the case after being found guilty of similar charges by a special court martial. Three other special court martials were pending.
In another case in al-Mamudia in August 2004, a detainee suffered second degree burns on the back of his hands.
The document said the detainee asked to use alcohol-based hand sanitiser liquid during a bathroom visit. A marine guard squirted some into the detainee's hands, but the excess formed a puddle on the floor.
"As the marine guard turned to dispose of the empty bottle, [the accused marine] lit a match and threw it into the puddle of hand sanitiser. The liquid ignited and the flames burned the detainee."
Bush says prisoner abuse is not
widespread in US-run jails in Iraq
The unidentified marine was found guilty of "assault by means likely to produce death or grievous bodily harm", and sentenced to 90 days confinement and a reduction in rank.
In another case cited in the documents, a marine guard shot and killed a detainee identified as Hamdan Shaiby on 29 March 2003.
"The investigation determined that the detainee attacked the
marine guard and the guard acted in self-defence when he shot the detainee that was lunging for the guard's service rifle," the document said.
Commenting on the revelations, Anthony Romero, ACLU's executive director, said: "This kind of widespread abuse could not have taken place without a leadership failure of the highest order."
Pentagon spokesman Lawrence DiRita said he had no information on the cases cited in the documents, which have not been previously disclosed.
He denied criticism by human rights groups that the military
often investigated abuses only after they had come under media scrutiny.
"Many of the cases that are being celebrated have had disposition already made," he said.
"And there may be a desire that disposition when it is made be publicised, but that's a different thing from saying that we are reacting to publicity."