TOKYO - A Vietnamese man held in a Japanese immigration detention center died on Saturday, six people told Reuters, drawing fresh attention to conditions in the country's detention system.

The man died at the East Japan Immigration Center in Ibaraki prefecture, northeast of Tokyo, according to activists, a lawyer and a detainee held at the facility. Two men died at the same facility in 2014.

The deceased man was named as Van Huan Nguyen and was one of more than 11,000 refugees that the country took in over the three decades to 2005 in the aftermath of the Vietnam War, according to two of the sources.

It was not clear for how long he had been detained or why.

Nguyen did not have relatives in Japan but had many friends, said one of the sources who had known him.

"He was a bright, fun person," she said. "I can't believe he died."

The Sankei daily reported that the East Japan Immigration Center said a guard had found a Vietnamese man in his forties lying unconscious in his cell alone early on Saturday.

The guard called an ambulance and the detainee was sent to hospital, where his death was confirmed about an hour and a half later, the Sankei said, citing the center.

The authorities would conduct an autopsy and investigate the cause of death, the Sankei said.

The East Japan Immigration Center and the Justice Ministry, which oversees immigration facilities, could not be reached for comment on Sunday.

Reuters was not able to independently verify the death or the identity of the deceased.

A Reuters investigation last year into the death of a Sri Lankan at a different facility in Tokyo revealed serious deficiencies in medical care and monitoring in the immigration detention system.

Since 2006, at least a dozen people have died while in immigration detention, including four cases of suicide.

Some detainees are held for months and even years with no clear knowledge of when they will be let out. Many develop depression and insomnia, detainees and psychiatrists have said.