An autopsy failed to answer whether a MARTA police officer shot a teenager while he was prone on the ground last week, the Fulton County chief medical examiner said Thursday.

Sometimes an autopsy can determine whether a shooting victim was shot while on the ground because the bullet goes through the body and strikes the pavement or the wound carries certain markings. That wasn't the case in the death of 19-year-old Joetavious Stafford, who was killed near the Vine City MARTA rail station Saturday, Chief Medical Examiner Randy Hanzlick said.

That means that once the Georgia Bureau of Investigation completes its inquiry, Fulton District Attorney Paul Howard could have to rely on witness statements -- which already are conflicting -- to decide whether to prosecute or clear Officer Robert Waldo in the homicide, Gwinnett County District Attorney Danny Porter said.

Murky police shootings can put district attorneys in a politically difficult position because they work closely with the police and can also face pressure from the public, Porter said. That, he said, is why they want good forensic evidence that can support at least one version of the events.

"I've been in the position that Paul Howard is in, and it is not fun," Porter said.

Sometimes investigators can determine whether a bullet had a downward trajectory, but Porter said that requires investigators to determine the victim's exact position when shot. "That is so rare it almost never happens," he said.

MARTA officials contend that Stafford was armed -- although they have not disclosed whether GBI investigators recovered a weapon. Stafford's brother Rodney Stafford said he saw Waldo shoot Joetavious Stafford while he was unarmed and then shot him twice more while on the ground, a charge that has already prompted criticism of the police.

Rodney Stafford told Channel 2 Action News, a reporting partner of The Atlanta Journal-Constitution, that his brother was involved in a melee near the MARTA station when somebody fired a gun in the air. He said his brother was fleeing when MARTA police officers ran to the scene, asking who had the gun. Stafford said his brother stopped and raised his hands before he was shot.

The autopsy showed that Joetavious Stafford was shot once directly in the back, while a second shot to the back and one to the chest each showed a sideways bullet path, Hanzlick said. He said it is difficult to determine the sequence of the wounds, which could be a key finding in determining whether there was any wrongdoing.

"The body can move between shots, and that can give you a bunch of different scenarios," he said.

Frank Rotondo, executive director of the Georgia Association of Chiefs of Police, said interviewing witnesses during the first day or two of an investigation is critical to its credibility because memories are freshest then. He said it's also important to record or get written statements -- with any changes in the typed statements being in the witnesses' own handwriting -- to ensure the statements stand up to scrutiny.

"If this goes to trial in two years, you really need those witnesses' statements to be fresh statements," he said. "The witnesses may be subject to a lot of pressure from the local community to modify their statements."

Rotondo said the reason a GBI investigation often takes weeks to complete because of the wait for laboratory results on forensic evidence or in determining whether drugs or alcohol were involved. The GBI does not make an official determination whether the shooting was justified but leaves it to the district attorney, GBI spokesman John Bankhead said.

Porter said tying a weapon to Stafford would be critical to the standard justification for a shooting that the victim presented a danger. But if Stafford was surrendering or if Waldo identified the wrong man as the gunman, Porter said, it would make the decision on whether to indict much more difficult.

“When you have an officer who is acting in good faith and makes a mistake, that makes it a tougher decision for the district attorney,” Porter said.