Prosecutors said on Thursday there was nothing Hungarian authorities could have done to prevent the deaths of 71 migrants in a lorry found by an Austrian motorway at the height of Europe's migrant crisis in 2015.
The refugees from Syria, Iraq and Afghanistan - 59 men, eight women and four children - were crammed into the back of a small lorry by people smugglers and suffocated on the journey from Hungary.
The case highlighted the perils of the journey that more than a million migrants undertook as they fled conflict and economic hardship in the Middle East and Africa, causing a humanitarian crisis and political strife in Europe.
German daily Suddeutsche Zeitung reported on Wednesday that Hungarian police had intercepted and recorded mobile phone conversations among the traffickers who drove the lorry, but acted too late to stop it.
"It is incorrect to assume Hungarian authorities could have prevented the deadly crime considering its extremely rapid course," prosecution spokesman Gabor Schmidt wrote in a response to questions from Reuters.
He said Hungarian police were working to disrupt human trafficking networks that ferried migrants, often in poorly ventilated refrigerator trucks, across international borders.
Schmidt said before the deadly incident police had no indication the migrants would be in mortal danger inside the trucks and focussed on mapping the networks.
He said investigators had tapped the phones of the leaders of the trafficking ring that caused the deaths of the 71 migrants from Aug. 13, 2015 - two weeks before the deadly journey - trying to map traffickers in an earlier case.
Suddeutsche Zeitung detailed some of the conversations based on police records it obtained. In one instance, the Afghan ring leader explicitly forbids the driver to open the lorry's door despite loud banging and cries for help from the inside.
"You should in no way open the door," he says. "If people die in there unload them in a forest in Germany."
As this conversation took place in the early hours of Aug. 27, 2015, no one was listening in on the Hungarian side.
"Because there had been no data about transports threatening human life... we could not shorten the period of the wiretap (and move in on the traffickers sooner)," Schmidt wrote. There was "no reason or capacity" to listen to the conversations live.
He said the languages spoken - Pashto, and Bulgarian and Serbian dialects - posed additional difficulties.
"All of that meant the Hungarian authorities could only translate and analyse the wiretaps after the transport that led to the deaths of the 71 migrants," he said, adding authorities had observed all relevant Hungarian laws.
Eleven men have been charged in connection with the deaths. Their trial is due to start next week.