The Nikkei article says:
On 29th March, Ushiku Police told that a male aged 33 who had been detained at Higashi Nihon Nyukoku Kanri Centre (Ushiku Immigration/Detention Centre) collapsed and then died. Postmortem is to take place.
According to the police, the man suddenly collapsed while eating meal in his room (in fact it's a "cell" rather than a "room" - ed) around 19:50, 28th March. He was hospitalised (not clear when - ed) but pronounced dead around 15:00, 29th March. The Immigration Centre got in touch with the police around noon, 29th.