DURING the contentious 1982 murder trial of Mumia Abu-Jamal, a critical part of the prosecution's case for sending him to death row was the testimony of two witnesses who said they saw Abu-Jamal repeatedly shoot the prone, defenseless Officer Daniel Faulkner while standing over him.

But this testimony crucial to the prosecution portrayal of Abu-Jamal as a cold-blooded assassin, raises a question: Why, if he fired four bullets down at Faulkner, only hitting him once in the head, was there no evidence in the sidewalk around the officer's body of the bullets that missed?

Now two independent journalists (the authors) have raised further questions about that troubling lack of bullet-impact evidence by doing something neither the defense nor prosecution attempted. A similar .38-caliber revolver and similar high-velocity, metal-jacketed bullets were fired from a similar distance into a slab of old concrete sidewalk similar to that at the scene of the Dec. 9, 1981, shooting on the south side of Locust just east of 13th Street in Center City.

The results show it's impossible to fire such a gun into a sidewalk without the bullets leaving prominent, clearly visible marks. Yet the prosecution case had Abu-Jamal performing that exact miracle, missing the officer three times without leaving a trace of his bad marksmanship.

So where are the missing bullet marks? The police crime-scene photos presented to the jury didn't show any, and police investigators in their reports didn't highlight or even mention any bullet marks on the sidewalk around the slain officer's body.

Gunshots fired into a sidewalk would also produce fragments either from bullets or material blown out of the sidewalk, yet neither the coroner's report nor a police report on the analysis of Faulkner's uniform makes any mention of bullet or sidewalk fragments on the slain officer.

We provided a crime-scene photo and our test photo to veteran NASA analyst Robert Nelson. Using enhancement techniques he applies to Cassini images from Saturn, he found no bullet marks. Nelson says crime-scene photos should have shown visible bullet marks comparable to the test photo had Abu-Jamal fired as the witnesses testified.

This raises the question: Were those two key witnesses lying?

Appellate courts have consistently upheld Abu-Jamal's conviction, but no court has considered this contradiction between witness testimony and ballistics tests showing it to be impossible to shoot into a sidewalk without leaving marks.

The D.A.'s office declined to respond to four questions regarding the test results. A spokesman curtly dismissed the test as yet another instance of "misconceptions" regularly raised by those lacking knowledge of "the entirety of the record."

The Abu-Jamal case is back in the news with two documentary films about the matter premiering here this week. Many discrepancies in the case cry out for a retrial to resolve questions that challenge the verdict and sentence.

Dave Lindorff is an award-winning investigative journalist and author of "Killing Time: An Investigation into the Death Penalty Case of Mumia Abu-Jamal" (Common Courage Press, 2003). He is also a founding member of the online newspaper ThisCantBeHappening! (www.thiscantbehappening.net). Linn Washington, a professor of journalism at Temple University, has followed the Abu-Jamal case since it happened.