TOKYO (Reuters) - A Vietnamese man who died in solitary confinement at a Japanese immigration detention center, raising questions about monitoring of detainees, died from a stroke, a Justice Ministry official said on Monday.

FILE PHOTO: An inside view of a shared cell is seen at the East Japan Immigration Center in Ushiku, Ibaraki prefecture, Japan, March 19, 2015. REUTERS/Yuya Shino/File Photo

The death of Nguyen The Hung last month at the East Japan Immigration Centre in Ibaraki prefecture, northeast of Tokyo, took the toll in Japan’s detention system to 13 since 2006.

The deaths have provoked criticism from activists and a watchdog overseeing the centers over medical care and monitoring of detainees.

An autopsy found that Nguyen died of a subarachnoid hemorrhage, the official said, requesting anonymity. The autopsy was carried out on Wednesday, a separate person with direct knowledge of the matter said.

While full details of Nguyen’s medical condition and treatment at the center remain unclear, fellow detainees said the Vietnamese had repeatedly told guards he was suffering from pain in his head and neck after his arrival.

Nguyen was prescribed painkillers by a doctor at the center, the detainees said in a letter, only for guards to ignore his later complaints of pain as he was held in a solitary cell and tell him to be quiet.

Tooru Tsunoda, a doctor and vice-chairman of a watchdog overseeing conditions at the center, told Reuters that although strokes are hard to detect, guards may have missed symptoms of Nguyen’s illness.

“They may have been unable to detect the stroke by only monitoring Nguyen in the solitary cell,” Tsunoda told Reuters.

Nguyen, born in 1969, arrived in Japan in 1998 to seek asylum, a Vietnamese nun told Reuters. He was one of more than 11,000 refugees Japan took in during the aftermath of the Vietnam War.

A Reuters investigation into the death of a Sri Lankan held in a solitary cell at a Tokyo detention center revealed serious gaps in medical care and checks on ill detainees.