The parents of Otto Warmbier are suing North Korea for over $1 billion, saying their son died as a result of being tortured while in captivity.

The suit, which was originally filed in April, alleges that North Korea "tortured him (Warmbier), kept him in detention for a year and a half without allowing him to communicate with his family, and returned him to them in a non-responsive state and brain dead."

The college student from Ohio was on a tour in the communist country when he allegedly stole a propaganda sign from a hotel. He was arrested in January 2016 and held captive by North Korea for more than 17 months.

He died from severe brain damage after he was repatriated to the U.S. in June.

The court documents filed in October reveal that Warmbier’s parents are seeking $1.05 billion in punitive damages and about $46 million for the family’s suffering.

North Korea has repeatedly denied the accusations that he was tortured.

The Associated Press contributed to this report.