U.S. troops who abused Iraqis at Abu Ghraib prison did it "just for fun," a military investigator testified on Tuesday in a hearing for a female soldier photographed holding a naked Iraqi on a leash.

Pfc. Lynndie England, visibly pregnant, appeared on the opening day of the military-court hearing, which will determine whether she will be tried for the prisoner abuse that outraged the world.

England, dressed in camouflage uniform, black boots and beret, entered the courthouse moments before the hearing began, ignoring dozens of media cameras and reporters.

Chief Warrant Officer Paul Arthur, the lead investigator into the Abu Ghraib abuse, told the court England said in a sworn statement in January that one of her superiors, Spc. Charles Graner, put the leash on the naked Iraqi prisoner and told her to pose for the infamous photograph.

But in an effort to shoot down defense claims that the abusers were acting on orders from above, Capt. Crystal Jennings, the lead prosecutor, asked if Arthur had determined why the troops had abused the prisoners.

"Basically it was just for fun ... and to vent their frustration," Arthur said.

The two witnesses said England never raised any objections to the treatment of the prisoners.