BUCKS COUNTY, Pa. (CBS) — The parents of one of four men killed on a Bucks County farm have filed a wrongful death lawsuit.

Robert Ross, the attorney for Mark Potash and Aimee King, the parents of Mark Sturgis, filed a lawsuit against Cosmo DiNardo’s parents, claiming they neglected to safeguard their gun while knowing their son has serious mental issues.

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“According to the criminal complaint, he was, Mark Sturgis was, shot with a gun owned by DiNardo’s mother,” said Ross.

Ross says this could have been avoided if the gun was locked up safely.

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“In light of Cosmo DiNardo’s past, his history of being involunarily committed, of having run-ins with the law, of having been arrested, he somehow had access to that gun,” said Ross.

DiNardo and his cousin, Sean Kratz, have been charged in the murders of Sturgis, Jimi Patrick, Dean Finocchiaro, and Tom Meo. Their bodies were found in July on a farm in Solebury owned by DiNardo’s parents.

DiNardo and Kratz pleaded not guilty to the murders last week during a formal arraignment.

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Authorities say DiNardo admitted to killing the men and burying their bodies and as part of the original agreement with prosecutors he would not face the death penalty. Now, the Bucks County District Attorney’s Office has filed court documents giving them the option to seek the death penalty, if DiNardo does not continue to cooperate.