An Australian journalist detained in China for weeks is being held on national security grounds, China has said.
Cheng Lei, a presenter for China Global Television Network (CGTN), is suspected of "criminal activity endangering China's national security".
She has been detained since 14 August.
The announcement comes after the last two journalists working for Australian media in China flew home to Sydney after a five-day diplomatic stand-off.
The Australian Broadcasting Corporation's (ABC) Bill Birtles and the Australian Financial Review's (AFR) Mike Smith landed in Sydney on Tuesday.
Chinese authorities questioned both men before their departure. Mr Birtles told the BBC he was questioned about Ms Cheng.
Relations between Australia and China have deteriorated in recent years.
There had been allegations of Chinese interference in Australian society in the past, but ties worsened after Canberra backed an international inquiry into the origins of the coronavirus pandemic.
The Foreign Correspondents' Club of China said on Monday a record 17 foreign journalists had been expelled from the country in the first half of 2020.
What happened to Cheng Lei?
An Australian citizen based in Beijing, Ms Cheng is a high-profile, respected business journalist for English-language channel CGTN.
In August she suddenly disappeared from television and cut off contact with friends and relatives. China eventually announced she was being held under "residential surveillance" in an unknown location.
No charges were announced at the time. But now foreign ministry spokesman Zhao Lijian said the government detained her on "national security grounds", saying authorities had taken "compulsory measures" against her. An investigation is now under way, he added.
The spokesman gave no details about the accusations, and it is unclear what "criminal activity" she is alleged to have committed.
But at the time of her detention some believed her Australian nationality may have been a factor, amid increasingly poor relations between Beijing and Canberra.
What about the other Australian journalists?
Mike Smith and Bill Birtles left China on Monday after being interviewed by Chinese authorities.
Birtles told the BBC that Australian diplomats advised him last Monday that he should leave China immediately.
"They'd received some sort of warning or advice, they never specified what," said the journalist.
He was then booked on a flight due to leave Beijing last Thursday. But the situation escalated on Wednesday at midnight when six Chinese police officers and a translator visited his apartment as he held farewell drinks with friends.
He said the police did not detain him, but informed him that he was involved in a "national security investigation," and that he could not leave the country.
He contacted Australian consular officials the next morning, who collected him and took him to the Australian embassy, where he spent the next four days.
He was later interviewed by Chinese police in the presence of Australia's ambassador to China, Graham Fletcher. Mr Birtles said he was questioned about Cheng Lei.
"I know her, but not particularly well, it didn't seem like I would be the most logical person to interrogate if you wanted evidence about her case," he said.
Mr Birtles said he was also asked about sources he used to report on Hong Kong's national security law, and on Australia's relations with China.
"There wasn't any real rigorous effort to extract any evidence or anything that could really be used in any case," he added.
Mike Smith, who is based in Shanghai, also received a visit by police - prompting him to go to the Australian consulate there. He, too, was questioned over Ms Cheng, the AFR reported.
The pair were both allowed to leave the country after their interviews by police.
Why are Australia-China ties strained?
The countries rely heavily on each other for trade, but their relationship has soured in recent years, amid allegations of Chinese interference in Australian society.
Ties have deteriorated further this year after Canberra backed an inquiry into the origins of the coronavirus.
In actions widely viewed as retaliatory, Beijing has since imposed restrictions on Australian exports such as beef, barley and wine. It has also warned Chinese tourists and students about racism in Australia.
In July, Australia warned its citizens they may face "arbitrary detention" in China - a travel warning that remains in place.
Canberra has also expressed concerns about human rights in Xinjiang and Hong Kong, and urged China to release detained Australians such as Yang Hengjun, a democracy activist who has been held for 19 months without trial.