This is a story about the gun that killed Sgt. Patrick McDonald last year after a car stop, street chase and shoot-out, and how it came to Philadelphia.

The weapon - a Taurus, Model PR 145 Pro, a .45-caliber semiautomatic pistol - was found clutched in the hand of Daniel Giddings, 27, released from state prison 33 days before the Sept. 23, 2008, murder.

McDonald, 30, a highway cop with eight years' experience, stopped Giddings' car, and the ex-con took off. He turned back and fired the fatal shots at McDonald on 17th Street near York. Giddings was then fatally shot by another officer.

Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives then traced the murder weapon.

Yesterday, ATF agent Adam Cameron testified in federal court that the weapon had been manufactured in Brazil and imported by Taurus International through U.S. Customs in Miami.

The pistol ended up in the Lancaster Pawn Shop, in Lancaster, S.C., where the Taurus and another weapon were bought Nov. 13, 2003, by bricklayer Jason Mack, 30. Mack, of Sumpter, S.C., admitted on the witness stand that he has smoked marijuana "five or six" times a day for the past 15 years.

"I like guns," testified Mack, who estimated that he had owned about seven guns, including a rifle for hunting.

Mack was arrested and charged with making false statements regarding his drug use on ATM purchase forms, and pleaded guilty to illegally owning firearms. On June 1, he was sentenced to three years in prison; he has served six months.

In the spring of 2006, Stephen "Doe" Lashley, 30, of West Philadelphia, drove to Lancaster, S.C., to visit his friend, Faheem Berry, who had moved from Philly in December 2005. The two had known each other since 2001.

Berry testified that during the visit, Lashley allegedly brought marijuana in one-inch glass jars and they were smoking it with Mack.

Berry testified that he overheard Lashley ask Mack where he could buy guns, and Mack offered to take him to the Lancaster Pawn Shop, about five minutes away.

When they came back, Lashley allegedly put "one or two" guns in a gym bag and put that into the trunk of his vehicle, Berry said. Lashley allegedly paid Mack $75 and marijuana for the guns.

Lashley made two more trips to allegedly buy other weapons through Mackthat were brought back to Philadelphia, Berry said.

Lashley, on trial in federal court for illegally possessing firearms and transporting them to Pennsylvania, was prohibited from owning firearms as a result of a 1999 felony narcotics conviction.

But here's the mystery:

The Taurus ends up in Philadelphia in 2006. Giddings is using it 33 days after he's released from Pennsylvania state prison in 2008.

But Lashley isn't talking about what happened with the weapon in that two-year period.

And Assistant U.S. Attorney Arlene Fisk, who is prosecuting Lashley, said: "There is absolutely no evidence linking the defendant with Giddings."

Yesterday, McDonald's sister Megan attended Lashley's trial to follow the path of the weapon that killed her brother. Now in Drexel Law School, McDonald said that she wants to become a prosecutor.

"I love Philadelphia. I would like to contribute to keeping it safe the way my brother did."