Yonhap News agency quoted Seoul prosecutors as saying on Tuesday the hospital had testified it gave Park the shot but did not realise it was against World Anti-Doping Agency regulations.
The report said Park had already been questioned by prosecutors, and that the doctor who gave him the injection could face charges of negligence.
A two-time world champion, Park became the first South Korean to win an Olympic swimming medal when he powered to gold in the 400 metres freestyle at the 2008 Beijing Games.
His wholesome, clean-cut image and poster-boy looks have made him one of the most celebrated athletes in South Korea, where he is known affectionately as 'Marine Boy', and while his performances in the pool have dipped in recent years his popularity has never wavered.
The smiling face of endorsements for milk, headache pills, air conditioners, and communications equipment, Park was also a goodwill ambassador for the 'Dynamic Korea' promotion aimed at boosting Korea's image abroad.
Under the strict rules applied by doping authorities, Park could still face punishment even if he was unaware that he had been injected with a banned substance, and a lengthy ban could derail his hopes of swimming at the 2016 Rio Olympics.
An official from the Korea Swimming Federation, who asked not to be named, told Reuters that the test had been conducted in early September, before the Asian Games.
The federation and Park had been informed of the result in late October or early November, the official said, adding that the swimmer would have to attend a hearing with swimming' s world governing body FINA on Feb. 27 to answer the doping charge.
The official said other details would be revealed at a news conference after the prosecution had wrapped up its probe.
'MORE SHOCKED THAN ANYONE'
Park's agency, Team GMP, said the swimmer had repeatedly asked the hospital if the injection contained illegal substances but had been assured it was safe.
Park had been offered free treatment at a local hospital two months ahead of the Sept. 19-Oct. 4 Asian Games in Incheon, the South Korean city west of Seoul which named its aquatics venue after the swimmer.
"As a world-class swimmer for the last 10 years, Park Tae-hwan hasn't taken so much as cold medicine, that's how careful he's been due to concerns about doping problems and illegal substances," Team GMP said in a statement.
"Park is more shocked by this result than anyone else."
The statement said the 25-year-old had passed several doping tests conducted during the Asian Games.
South Korean athletes have been relatively free of drugs scandals, though top badminton player Lee Yong-dae was almost suspended for the Asian Games after administrative errors led to him missing doping tests.
A one-year ban imposed by the Badminton World Federation was wiped out after the country's governing body said it had failed to keep the federation apprised of the player's whereabouts.
The news also comes just months after Park's great rival, China's Sun Yang, was found to have failed a doping test.
Sun, winner of two gold medals at the 2012 London Olympics, served a three-month suspension in 2014 after testing positive for the banned stimulant trimetazidine during the national swimming championships last May.
The China Anti-Doping Agency could have imposed a longer ban but said it decided on three months because Sun had been given medication, normally used to combat angina, by a doctor to treat a heart issue and was unaware it had been added to WADA's banned list this year.
(Reporting by Sohee Kim and Kahyun Yang; Editing by Sudipto Ganguly)
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