Jason Mack is scheduled to be arraigned in federal court in South Carolina.
Investigators say he lied on a federal form about his drug use, in order to purchase a gun.
That somehow ended up in the hands of Daniel Giddings, the man accused of killing Officer McDonald.
A viewing takes place for Officer McDonald on Monday night at the Givnish Funeral Home in Northeast Philadelphia. A second viewing will be held Tuesday morning at the Cathedral Basilica of Saints Peter and Paul. A mass of Christian burial follows at noon Tuesday.
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