A US Army dog handler accused of abusing inmates at Iraq's Abu Ghraib prison in 2003 and 2004 has gone on trial at a military court in Maryland.
Sgt Cardona is the second dog-handler charged over Abu Ghraib
Sgt Santos Cardona, 32, faces several charges including dereliction of duty, conspiracy and assault for allowing his dog to intimidate two detainees.
His defence team is expected to argue that his use of the dog was condoned.
A seven-member jury was selected, but further proceedings were suspended when a witness failed to appear.
The witness, Megan Ambuhl Graner, is a former Abu Ghraib guard who pleaded guilty in November 2004 to failing to prevent or report maltreatment of prisoners.
She is married to Charles Graner, currently serving a 10-year prison sentence as the ringleader of the group of soldiers who abused prisoners at Abu Ghraib.
Mrs Graner had not answered a subpoena to appear before the court, a prosecution lawyer said. The prosecution would seek a federal court writ to secure her appearance, he said.
The court is expected to hear testimony from the highest ranking officer ever to take the stand in any of the prosecutions arising from the abuse at Abu Ghraib.
Images of Abu Ghraib abuse have shocked the world
A senior army officer, Maj Gen Geoffrey Miller, has been ordered to testify on US interrogation policies.
Sgt Cardona's lawyers say they will press him for information about a trip he made to Iraq to advise US officials on how to get better intelligence from detainees.
They say that shortly after his trip, military dogs were shipped to Abu Ghraib and approved for use in interrogations.
The prosecution, however, say there is no evidence to suggest that orders came to use dogs in this way.
The trial is taking place at Fort Meade military base. If convicted, Sgt Cardona could face up to 16 years in jail.
Of the two detainees Sgt Cardona is accused of intimidating with a dog, one was an Iraqi general, the other was believed to be an al-Qaeda operative.
A total of 10 US soldiers have already been found guilty of abuses at Baghdad's Abu Ghraib prison.