evidence sufficient to support a finding

On the evening of April 15, 1989, two white Air Force police officers named Jane Smith and Tony McCormack were walking back to their dormitories at Fort Dix, New Jersey, after seeing a movie. They sat under a glass-enclosed bus shelter to avoid light rain. A black male wearing a wool cap and tan nylon jogging suit entered the shelter, drew a small silver handgun, robbed McCormack of his wallet containing an unsigned $100 money order, and forced Smith to perform fellatio on him while threatening to shoot McCormack. After the assailant fled when a car drove by, the victims contacted military police. Investigator Christine Amos suggested they look at a wanted board containing eight posters with composite sketches and photographs. McCormack immediately identified a photograph of defendant Richard Stevens, and Smith agreed it resembled the attacker though he appeared heavier in the photo. Smith then went to Walson Army Community Hospital where a doctor administered a rape crisis kit and attempted to secure semen samples from her saliva and clothing, producing three glass slides. The FBI laboratory later performed serological testing on one slide containing sufficient semen, which consumed all material on the slide and prevented DNA testing. A consent order had directed the FBI to preserve samples if possible after its own tests so Stevens could conduct DNA testing. The government made remaining materials available to the defense nearly six weeks later, but testing proved inconclusive. Stevens was indicted on charges of aggravated sexual assault and robbery within the special territorial jurisdiction of the United States. Stevens's first trial ended in a mistrial after the jury deadlocked. At the second trial in March 1990, the district court permitted expert testimony on cross-racial identifications, weapon focus, and stress but excluded testimony on the lack of correlation between confidence and accuracy in identifications. The court also excluded testimony from Tyrone Mitchell, who had been robbed at gunpoint three days later at Fort Dix in a similar manner; Mitchell did not identify Stevens, and the fruits of both crimes ended up near Fort Meade, Maryland. The jury convicted Stevens on both counts, and the district court sentenced him to 168 months of incarceration. Stevens appealed, challenging the destruction of the semen sample, the wanted board identification, the admission of pretrial services testimony, the limits on expert testimony, and the exclusion of the Mitchell evidence. The Third Circuit reviewed the record developed at the Wade hearing, the Brady motion hearing, the Downing hearing on expert testimony, and the trial proceedings.