Tests on part of a letter opener that prosecutors say a priest used to kill a nun 26 years ago showed no DNA evidence, an expert with the state crime lab testified Thursday.

A piece of the bloody altar cloth found in the hospital chapel where Sister Margaret Ann Pahl's body was discovered also was tested and was found to contain only the nun's DNA, Diane Gehres testified.

The testimony came at the trial of the Rev. Gerald Robinson, 68, in the slaying of Pahl. Robinson was the hospital chaplain when Sister Pahl was found stabbed to death and choked on the day before Easter, 1980. She was found in the chapel where both worked, but authorities have not disclosed a motive in her killing.

On Wednesday, another DNA expert testified that the nun's underwear had traces of DNA that did not come from her or Robinson.

The DNA found on Sister Pahl's underwear was likely from a man, but tests did not link the sample to Robinson, said Cassandra Agosti, a forensic analyst with the state's crime lab.

Prosecutors said the trace of DNA was so small that tests showing it came from a man might have been wrong, or the sample could have been left by someone who moved her body after the crime.

Agosti also tested the letter opener and found a substance that might have been blood. But she said she could not prove that.

A medical examiner testified on Wednesday that bloodstains on the altar cloth and the forehead of the nun could have come from a letter opener sword-shaped letter opener belonging to Robinson.

The altar cloth was found in the chapel where the nun's body was discovered. The letter opener was found by police in a desk belonging to Robinson.

The stains were a key reason why prosecutors decided two years ago to charge Robinson with killing Sister Pahl. The two worked together closely, and Robinson presided over the nun's funeral.

Medical examiner Paulette Sutton told jurors Wednesday that she looked at 18 stains and found that most could have been made by the letter opener. She said one faint stain showed what looked like the outline of the U.S. Capitol, which was on a dime-sized medallion on the letter opener.

A bloodstain on Sister Pahl's forehead also was a potential match with the letter opener, Sutton testified. Other stains on the nearly 10-foot-long altar cloth could have been made by the letter opener's ribbed handle, she said.

Under questioning from defense attorney John Thebes, Sutton said a pair of scissors could have left one of the stains found on the cloth. Investigators have said a pair of scissors were missing from the chapel.

Robinson was a suspect early on because he was near the chapel at the time of the death. He could get life in prison if convicted of murder.