The U.S. military was scrambling to establish the fate of King, who made an unauthorised crossing of the inter-Korean border into North Korea on Tuesday, throwing Washington into a new crisis in its dealing with the nuclear-armed state.
King's motivations for his high-stakes gambit remain unclear.
U.S. officials said he had finished serving time in detention in South Korea for an unspecified infraction and was transported by the U.S. military to the airport to return to his home unit in the United States, when he apparently decided to join a tour to the North Korean border.
King pleaded guilty to assault and destruction of public goods stemming from an October incident, and on Feb. 8 the Seoul Western District Court fined him 5 million won ($4,000), according to a copy of the ruling reviewed by Reuters.
Two U.S. officials, who spoke on condition of anonymity, said the soldier had been due to face disciplinary action by the U.S. military.
Reuters was not immediately able to ascertain whether the disciplinary action was linked to his conviction over damaging the police vehicle.
The Seoul court said on September 25 last year King punched a man in the face at a club several times but the case was settled.
Two weeks later, on October 8, police officers responded to a report of another altercation involving King, and tried to question him. He continued with his "aggressive behaviour" without answering questions from police, according to the court document.
Police placed him in the backseat of their patrol car where he shouted expletives and insults against Koreans, the Korean army, and the Korean police, the ruling said. During his tirade, he kicked the vehicle's door several times, causing about 584,000 won in damages, the ruling said.
The court said the defendant had admitted to the charges, had no previous criminal record, and paid 1 million won to fix the vehicle, citing reasons in favour of him in the sentencing.
A spokesman for U.S. Forces Korea (USFK) declined to confirm whether King had been in South Korean or U.S. military detention.
One of the lawyers who represented him at the time told Reuters King had spent time in U.S. military detention in Pyeongtaek since the October case.
The lawyer, who declined to be named due to the sensitivity of the matter, said he was unaware of the status of King's custody or whereabout after February.
King's other lawyers listed in court documents were not immediately available for comment.
King's mother, Claudine Gates, told ABC News she was shocked at the news her son had crossed into North Korea.
"I can't see Travis doing anything like that," she told the U.S. broadcaster.
($1 = 1,266.9100 won)
