An Egyptian appeals court on Saturday overturned the conviction of a policeman who was sentenced to 10 years in jail for the deaths of 37 prisoners from tear gas.
It also overturned suspended one-year sentences handed to three other officers over the August deaths of the prisoners, who were alleged supporters of ousted Islamist president Mohamed Morsi.
The 37 died after the tear gas was thrown inside their closed police van when they were being transferred to Abu Zaabal prison near Cairo.
Since Morsi the army ousted Morsi in July, the military-installed authorities have launched a crackdown on his supporters that has left more than 1,400 dead in street clashes and at least 15,000 in prison.
Hundreds have also been sentenced to death in speedy trials.
The officer given 10 years was the deputy head of the police station who oversaw the transfer.
The four officers were sentenced for manslaughter after the prosecution's investigation revealed they acted recklessly toward the victims.
On Saturday, the Appeals court ordered the case to be transferred to the general prosecution for further investigation.
The decision "means that the case is back to square one", human rights lawyer Amr Imam told AFP.
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During the first trial, a justice ministry expert said the truck used transporting the victims had a capacity of only 24 people, but was carrying 45 that day.
The interior ministry said at the time of the incident, which took place at the peak of the crackdown against Morsi's supporters, that police fired tear gas when the inmates rioted.
It came four days after security forces stormed two pro-Morsi sit-ins in Cairo, sparking clashes that killed hundreds.
In February, a court acquitted six police officers of killing 83 protesters in the Mediterranean city of Alexandria during the 2011 uprising against dictator Hosni Mubarak.
Nearly 850 people died during the 18-day uprising that toppled Mubarak, when protesters battled the then-despised police.
But in the past three years, blame for killings during the uprising has shifted to Morsi's Muslim Brotherhood as the police has been rehabilitated in public opinion.
Morsi himself and top Brotherhood leaders are on trial on charges that could incur the death penalty.
On Saturday, a court postponed until July 5 the verdict in the trial of Brotherhood leader Mohamed Badie and 37 others for inciting violence that killed two people last summer in the Nile Delta city of Qaliub.
But it sentenced to death 10 defendants who are on the run, and a final ruling on their cases is expected the same day they are reviewed.