Davis and Posey were killed around 2 a.m. on Nov. 15, 1972, as they were leaving their patrol car outside the borough hall. Prosecution witnesses linked Hamm, then a 28-year-old ex-convict from East Marlborough, to a rifle that was used in the murders. In his two-week trial in Dauphin County, Hamm suggested that the killings had been committed by a friend who had died in a 1973 car accident.
Since his conviction, Hamm has been working to get himself out of jail. He has filed three state post-conviction petitions and 14 federal petitions, said Deputy District Attorney Nicholas J. Casenta.
``This defendant has had his day in court,'' Casenta said yesterday in Judge Lawrence E. Wood's courtroom in West Chester.
But Hamm wants more time in court. He has hired prominent West Chester lawyer John J. Duffy to argue that he was convicted despite insufficient evidence and due to ineffective counsel.
Hamm, who is being held at a state prison in Huntingdon, appeared in court yesterday looking pale and wizened. He smiled slightly as Duffy rose to ask the judge for another post-conviction hearing.
Duffy focused his comments on the murder weapon linked to Hamm. Four days after the shooting, the state police, assisted by about 100 volunteer firefighters, searched a field near Hamm's house.
They discovered a .308-caliber rifle that ballistic experts determined had been used to kill Davis. The bullet fragments recovered from Posey's body were too badly damaged to determine whether they were fired by the same rifle, but one expert said they were fired from a similar rifle.
The prosecution, headed by then-District Attorney William H. Lamb, introduced evidence linking Hamm to the murders. Almost three years before the killings, the police had seized four weapons from his home, including the .308 rifle. Hamm also was seen practicing with the rifle prior to the shootings.
In his petition to win Hamm a new hearing, Duffy argued that ``at best, the prosecution proved that a weapon once taken from the Hamm household in a police search was used to kill one of the two police officers found dead.''
In court yesterday, Duffy added that the 1970 search for weapons was declared illegal and unconstitutional. When evidence from that search was then used at the trial, Duffy said, ``that was poison.''
To be eligible for post-conviction relief, a defendant must prove that his complaints were not litigated in the past. The issue of insufficient evidence was previously appealed to the state Supreme Court, Casenta said. ``Once an issue has been addressed or decided, you can't come back just because you have a new theory,'' he told the judge. ``I can't stress to you enough - this case is done.''
But Wood replied that he would rather err on the side of caution, ``allowing everybody to express his complaint.'' He asked both sides to appear before him again in two weeks.
For members of Kennett Square's police department, as for others involved in the case, the specter of a new trial is emotionally draining.
``On June 18, I'm bringing every available officer to the courtroom,'' Chief McCarthy said in a telephone interview after the hearing. ``As far as I'm concerned, [Davis and Posey] are still members of the police department. We owe their families our support.''