The Bureau of Alcohol, Tobacco, Firearms and Explosives says the gun was found at the crime scene but had not been fired.
The agency says a semiautomatic was used to kill 39-year-old Philadelphia Police Sgt. Stephen Liczbinski on Saturday as he investigated a bank heist.
Police continue to search for 33-year-old suspect Eric Floyd. One alleged accomplice is in custody and the other was killed.
Swigart is being held in Perry County on $500,000 bail, charged with two felony theft counts.
