A medical examiner testifying in the assault trial of a former CIA contractor said Friday that an Afghan detainee the defendant interrogated probably died from a beating, but a defense expert said there's too little information to know the cause of death.

David Passaro, a former Special Forces medic working in Afghanistan as a CIA contractor, is accused of beating Abdul Wali during a 2003 interrogation about rocket attacks on a remote base housing U.S. and Afghan troops. Defense attorneys have said Passaro never hit Wali.

Dr. Reinhard Motte, a medical examiner from Fort Lauderdale, Fla., testified that Wali probably died from injuries sustained during a beating described by soldiers who witnessed the interrogation. Several Army paratroopers testified this week they saw Passaro hit Wali repeatedly with a metal flashlight and kick him in the groin.

Click here to visit FOXNews.com's Law Center

"If I were to write a death certificate on Abdul Wali, I would write blunt force abdominal and pelvic injuries," Motte said.

But after the prosecution rested Friday, a medical examiner called by Passaro's attorneys said he could not determine the cause of death based on 12 photos of Wali's body and descriptions of his treatment.

"I feel that would be insufficient information to make such a ruling," Navy Cmdr. Craig Mallak said, adding that some of the bruises and abrasions shown in the photographs could have come from attempts to revive him.

An autopsy was never conducted. CIA investigator Fred Klare testified Friday that Wali's father wouldn't even tell the Americans where in Afghanistan his son was buried.

Another prosecution expert, Dr. Anthony Meyer, said Wali's most serious injuries "would be the two described kicks to the perineum, the area between the thighs, and the hit to the abdomen with the flashlight."

Meyer, the chief of surgery at the University of North Carolina hospitals, said during cross-examination that he couldn't give an exact opinion about what led to Wali's death. Upon further questioning by prosecutors, he said Wali "most likely died from sepsis infection or blood loss progressively over the course of two days."

Passaro, 40, of Lillington, is charged with four assault counts and could be sentenced to up to 40 years in prison if convicted. He is not charged with Wali's death.

Passaro is the first American civilian charged with mistreating a detainee during the wars in Iraq and Afghanistan. He is standing trial in his home state under a provision of the USA Patriot Act allowing charges against U.S. citizens for crimes committed on land or facilities designated for use by the U.S. government.

The trial is set to resume on Tuesday.