A cigarette butt is the only piece of DNA evidence that might link Terrell Yarbrough to two college students murdered 10 years ago, said an expert who reviewed more than 200 pieces of evidence in the case.

However, former FBI serologist and DNA expert Julie Kidd testified yesterday in Washington County court that multiple samples of blood evidence instead implicated a co-defendant, Nathan "Boo" Herring, of Steubenville, Ohio.

Though the testimony came from a prosecution witness, it bolsters claims from Mr. Yarbrough's defense team that Mr. Herring was the triggerman in the Memorial Day 1999 shooting deaths of Aaron Land, 20, of Philadelphia, and Brian Muha, 18, of Westerville, Ohio.

Mr. Yarbrough's lawyers have painted their client as a borderline mentally retarded sidekick of Mr. Herring, but prosecutors maintain that Mr. Yarbrough participated in the slayings and should be equally culpable.

They are seeking a first-degree murder conviction and the death penalty.

Police theorize that Mr. Yarbrough, 29, of East Liberty, and Mr. Herring, 28, robbed and kidnapped Mr. Land and Mr. Muha at their off-campus apartment near Franciscan University of Steubenville.

They are accused of driving the students several miles into Robinson, Washington County, in Mr. Muha's Chevrolet Blazer, marching them up a steep embankment along Route 22 and shooting them to death.

Though Mr. Yarbrough and Mr. Herring were convicted nine years ago in Ohio of the murders, the Ohio Supreme Court ruled that the pair should be retried in Pennsylvania, where the students' bodies were found four days after they went missing on May 31, 1999.

Mr. Herring faces a retrial soon, but in the three days since Mr. Yarbrough's trial started, he has been featured prominently, especially in testimony involving guns, ammunition and other physical evidence.

Ms. Kidd testified that although deep-tissue samples from Mr. Muha and Mr. Land were somewhat degraded due to decomposition, she was able to obtain samples from their parents to compare with blood evidence on 28 items.

Three jackets found in Mr. Muha's Blazer tested positive for Mr. Herring's blood, as did multiple blood drops in another stolen vehicle and on several ATM receipts from Mr. Muha's bank account.

A stolen bank card from the same account was found during a search of Mr. Herring's home, police said.

The cigarette butt with Mr. Yarbrough's DNA was found in the Blazer, and though Ms. Kidd could not absolutely declare him the source of the DNA, the chances the sample came from another person is one in 24 billion among the black population, and one in 81 million Caucasians.

To be definitively characterized as a "source of DNA," Ms. Kidd said a sample must have less than a one in 280 billion chance of being from a different source.

Mr. Herring's blood was found mixed with that of Mr. Land on a sample taken from the Blazer, Ms. Kidd said, and Mr. Land's blood was found on several gauze bandages in the rear of the SUV.

Defense lawyers have pointed to cuts and wounds on Mr. Herring's hands and shoulders as evidence that he beat the students, possibly pistol-whipping them.

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