The FBI forensic team stated that they found no evidence at the crime scene to indicate someone had been firing at Lt. Rivers, no weapon and no shell casings. However there were bullet impact marks on the rock and trees he was hiding behind, something that the forensic team would have found, just as Rivers finds when he returns to the location of the shooting. In fact the bullets would still be embedded in the trees, assuming someone didn't remove them as well.
Lt. Rivers is said to be a highly decorated Navy SEAL, in fact he was even awarded the Medal of Honor in his first appearance. Which was over two years ago, at this point he has been a SEAL for almost a decade, yet he still holds the rank of Lieutenant. With his service record and time in service he would be at least a Lieutenant Commander by now.