Transformers' Peter Cullen's cause of death revealed

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. His family asks that you honor his remarkable legacy by serving your communities and leading others with compassion, integrity, and loyalty and always remember 'be strong enough to be gentle.'" 

He is scheduled to be buried at Forest Lawn on September 9.

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, and continued in the role for decades across various Transformers media. 

His Megatron counterpart, longtime voice actor Frank Welker, paid tribute to their creative partnership in a statement. 

"Optimus Prime and Megatron were made for each other. After all, one needs the perfect hero against the epitome of evil to create great drama. Peter's portrayal of Prime was born of his own personal beliefs: honesty, loyalty, and always insisting on doing the right thing. This made it easy to be friends... I will miss my friend, but that voice and his heart and values will always be part of Optimus Prime." 

Welker is set to reprise Megatron and Soundwave opposite Cullen's final performance as Optimus in the upcoming Optimus Prime: Awakening.