The cause of death for Peter Cullen, the longtime voice of Optimus Prime in the Transformers franchise, has been revealed. According to a death certificate obtained by TMZ, Cullen died of cardiac arrest brought on by respiratory arrest, with the underlying cause listed as a malignant neoplasm of the lower lobe of the lungs.

He died on August 26 at his home in Toluca Lake, California, at age 85. His family confirmed his passing in a statement at the time, saying, "Peter Cullen passed away peacefully, surrounded by his loving family, and held in the hearts of his many friends and countless fans worldwide."

Cullen originated the role of Optimus Prime in the original 1980s Transformers animated series before returning to voice the character in Michael Bay's live-action film franchise starting in 2007. He continued in the role for decades across various media.

His Megatron counterpart, longtime voice actor Frank Welker, paid tribute to their creative partnership in a statement, noting that Cullen's portrayal of Prime was born of his personal beliefs in honesty, loyalty, and integrity.