A 71-year-old man who dedicated a large part of his life to searching for his missing daughter has died, a civic group said Wednesday.

Song Gil-yong, whose daughter, Song Hye-hee, went missing in 1999, died Monday in a car accident, according to the local group dedicated to finding missing people here. The head of the group, Na Ju-bong, said in a media interview that Song had been suffering from deteriorating health, even showing signs of dementia in recent years.

Na described Song as "a man who showed me what it truly means to love his child." He said the deceased used to ask him to continue searching for his daughter in the event that he himself were to pass away.

Song had been on a frantic search for his daughter since she went missing on Feb. 13, 1999 after getting off the bus near her home in Pyeongtaek, Gyeonggi Province at around 10:15 p.m. Police had initially thought the then-17-year-old had run away from home and only launched an investigation three days after the incident, a move that had sparked criticism of the police.

The bus driver had testified that there had been a suspicious man who also got off at the same time as the girl. The suspect had been described to be in his 30s, wearing a hat that hid most of his face, smelling of alcohol and appearing not to be from the area.

Investigators failed to track down the suspect despite a large-scale search that followed, and the lack of surveillance cameras in the bus made things difficult. Police suggested the possibility that the victim had voluntarily run away from home but the family denied it, pointing out she had been a model student with top grades at school and with no feud with her family.

The case was closed after the statute of limitations for potential punishment -- which was at the time 15 years for murder -- expired in February 2014. The woman's whereabouts remains a mystery to this day.

Song Hye-hee was born on July 14, 1981, and would be 43 if she were alive today. She is 163 centimeters tall and is described as having a round face with relatively dark skin.

Despite the police closing the case, Song relentlessly searched for his daughter, travelling across the country to anyone who said they had seen Hye-hee. He hung up flyers and banners requesting information on his daughter all over the country, which had cost him his entire savings and left a substantial amount of debt. At the time of death, he had been subsisting on collecting and selling scrap materials.

He also had refused to change his mobile phone number out of concern that his daughter might someday call the number.

His wife suffered from depression and alcoholism after their daughter went missing, and took her own life in 2006, while clutching flyers for the girl.

Song had said in a past interview that he gave up on killing himself after his wife's death, saying he would dedicate his life to finding their daughter.