The "Veep" star said in a statement that the coroners had told him that a mixture of drugs clubbed with an unknown medical condition were responsible for her passing, reported Variety.
"We learned today the combination of drugs in Michelle's system, along with a condition we were unaware of, proved lethal," said Oswalt.
He further said that blockages, along with medications like Adderall, Xanax and pain medication Fentanyl claimed McNamara's life.
McNamar was crime writer best known for her website TrueCrimeDiary. She passed away at the age of 46 on April 21 last year.