An almost decadelong investigation into accused Khmer Rouge war criminal Ao An concluded on Friday, with a closing order – which will either indict or dismiss him – expected by June.

Ao An, also known as “Ta An”, faces charges of genocide of the Cham people, premeditated homicide and crimes against humanity, allegedly committed at a number of security centres and labour sites in Kampong Cham between 1975 and 1979.

An’s case has now been severed from Case 004, in which Yim Tith is still under investigation and will proceed as Case 004/02. The severance, court spokesman Hayat Abu-Saleh explained, was to ensure An’s right to an expeditious trial “with no delay”.

The prosecution and defence have 15 days to ask for further investigative action, she said, before the co-investigating judges begin drafting their closing order, which is expected by the end of June.

An’s international lawyer, Göran Sluiter, reiterated that his client is not one of those “most responsible” for the atrocities, meaning he is outside of the court’s jurisdiction and therefore the “investigations should be terminated”.

“[O]ur client maintains his innocence, and we are taking all necessary legal steps to challenge a closing order that might conclude that there is sufficient evidence against our client to send the case to trial,” he said in an email.

He added the investigations had lasted since 2008 and there “certainly has been undue delay in the investigations”.