Two former Nauru MPs claim deliberate delays by government officials led to the death of the former president Sprent Dabwido.
46 year old Mr Dabwido died in Australia in early May after going there for cancer treatment.
The former president and his supporters had said his health was compromised by the Nauru government's delays in issuing him a passport, needed so he could leave for treatment that was not available in Nauru.
Mr Dabwido was one of the group dubbed the Nauru 19, who had been facing charges over an anti-government protest in 2015.
Another member of the group, former MP Squire Jeremaiah, said he and former justice minister, Mathew Batsiua, filed a criminal claim over the delays leading to Mr Dabwido's death.
Mr Jeremaiah said they wanted to find justice for Mr Dabwido, adding that they had provided evidence to the police and been assured by them that it would be handed over for investigation.