TOKYO (Reuters) - A Vietnamese man who died in a solitary cell at a Japanese immigration detention center complained of pain throughout his detention for a week before his death, according to fellow detainees.

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 was the 13th in Japan’s detention system since 2006, a toll that has provoked sustained criticism from activists and a watchdog overseeing the centers about conditions prevailing there.

In a handwritten note seen by Reuters on Tuesday, six detainees said the man, Nguyen The Hung, repeatedly told guards he was suffering from pain in his neck and head after his arrival at the East Japan Immigration Center in mid-March.

An official at the center northeast of Tokyo, declined to elaborate on a statement issued on Monday saying that a Vietnamese man in his forties had been found unconscious there on Saturday and later pronounced dead.

A Vietnamese nun helping to arrange Nguyen’s funeral, Tam Tri Thich, initially told Reuters on Monday that the Vietnamese embassy in Tokyo had told her that Nguyen had killed himself at the facility in the prefecture of Ibaraki.

On Tuesday, however, she said she had misheard the information and that in fact the embassy had told her only that Nguyen had died suddenly.

The embassy did not immediately reply to Reuters telephone and email requests for comment.

Nguyen was prescribed painkillers by a doctor at the center last Wednesday, the detainees said in their letter, only for guards to ignore his later complaints of pain and admonish him to be quiet.

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 monitoring of people held in Japan’s immigration detention system.

The cause of Nguyen’s death has not been announced. The center and the country’s Justice Ministry, which oversees the detention centers, have said the authorities would perform an autopsy.

The East Japan Immigration Center held 297 detainees at the end of last year, according to the Justice Ministry.