STOCKHOLM/HONG KONG (Reuters) - Chinese agents have seized Swedish citizen and Hong Kong-based bookseller Gui Minhai from a Beijing-bound train while in the company of two Swedish diplomats, his daughter said.

Gui was taken away on Saturday by plainclothes police, Angela Gui told Radio Sweden, adding she is concerned for the safety of her father who was released from Chinese custody in October. His whereabouts since then have remained unclear.

Angela Gui said her father was being escorted by two Swedish diplomats to seek medical attention for a neurological disorder known as Amyotrophic lateral sclerosis (ALS).

“They were on the train for five hours, I think, and, at one of the stops before Beijing, there were about 10 men in plainclothes that came in and said they were from the police and just grabbed him and just took him away. And after that I haven’t heard anything,” Angela Gui said.

China’s foreign ministry did not immediately respond to a request for comment on the reported incident.

Gui Minhai was abducted in Thailand while on holiday in 2015, one of five Hong Kong booksellers who went missing that year and later appeared in mainland Chinese custody. The four others have returned to Hong Kong.

Chinese authorities said Gui, who published books on the personal lives of Communist Party leaders, was released in October last year after serving a sentence for a traffic offense in 2003.

Sweden’s foreign ministry said Foreign Minister Margot Wallstrom had summoned China’s ambassador over the issue.

The ministry would not comment on what had happened on Saturday, but an official said Sweden was aware and would be able to provide more information, possibly on Tuesday.

“The Swedish Government is fully aware of what happened on Jan. 20. Firm actions have been taken at a high political level and we have been in contact with Chinese officials who have promised us immediate information about his condition,” the official, Patric Nilsson, said.

Reuters was unable to contact Angela Gui immediately.