Australian documentary filmmaker James Ricketson has been questioned over his links to the political opposition in Cambodia.
Ricketson was arrested on June 3 and later charged with espionage.
Photographs in a government-aligned newspaper of Ricketson flying a drone over an opposition rally led to speculation his detention was linked to illegally operating the craft.
But a source close to the case, who spoke on condition of anonymity due to the sensitivity of the matter, said the filmmaker's arrest was not related to the drone.
"He was questioned about his links to [former opposition leader Sam] Rainsy," the source said.
The ABC understands Ricketson was taken to a Phnom Penh court for questioning on June 21 and is expected to return to court on Wednesday for further investigation.
Under the French-based legal system in Cambodia, an investigative judge carries out the ground work of a case.
Cambodian police have given mixed messages about why the 68-year-old Australian was arrested.
Ricketson was widely known to be making a documentary about Cambodian politics and spent years following Mr Rainsy.
The two appeared together on ABC radio in 2015, but the Cambodian National Rescue Party (CNRP) is now distancing itself from the controversial Australian.
"I am afraid I have no comment on this subject and I don't think anyone at CNRP knows much about James Ricketson," CNRP MP Mu Sochua said.
Emails to Mr Rainsy and a CNRP spokeswoman went unanswered.
Ricketson compared Cambodian PM to Darth Vader
Ricketson has been a vocal critic of Prime Minister Hun Sen, who has ruled Cambodia for 32 years.
In a 2014 blog post entitled The Dictator And The Democrat, he compared Hun Sen to the Star Wars villain Darth Vader.
The opposition won 46 per cent of votes in a recent local election and for the first time in decades will present a real challenge to the ruling party in next year's national vote.
Ricketson's campaigns have irritated many and sometimes brought legal action.
He has blogged about Australia's policy of resettling refugees in Cambodia and mounted a one-man sit-in protest at Screen Australia that resulted in a restraining order.
In 2014 and 2016, Rickestson was convicted in Cambodian courts of defaming two separate child protection organisations.
His recent arrest was reportedly made without a warrant and he was detained well beyond the legal 72 hours before being charged.
If convicted, Ricketson could face up to 10 years in jail.
A trial date has not yet been set.