CAIRO (REUTERS) - A Cairo appeals court on Saturday cancelled a prison sentence against a police officer convicted in connection with the deaths of 39 people last year during political violence, judicial sources said.

In one of the most controversial incidents since the army ousted Islamist President Mohamed Mursi last July, 39 men described by the Interior Ministry as Islamists, Muslim Brotherhood members and their supporters died during what the ministry said was an attempted prison break.

But the prosecutor's office later said its investigations showed they died while being transported to jail in an overcrowded police van into which tear gas was fired.

Appeals court judge Mohamed Amel on Saturday sent the case back to the prosecutor general and ordered a new investigation that could result in a new trial or in the case being dismissed, legal sources said.

According to the prosecutor's office, 45 people were crammed into a van made to carry 24 and subjected to tear gas fire. "This led to the death of 37 prisoners and the suffocation of two others," said its report issued last October.

An investigation by Britain's Guardian newspaper concluded that not all of those in the police van were Islamists. Some were not even Mursi supporters, it found, but had simply been rounded up in the chaos following the August 14 clearing of a pro-Mursi sit-in in a Cairo suburb.

At the original trial, a Cairo court sentenced Lieutenant Colonel Amr Farouk in March to 10 years in jail with labour on charges of involuntary manslaughter and extreme negligence. Three other policemen were given suspended sentences.

It was not immediately clear if Farouk would be released from jail. The prosecutor general's office was not immediately available for comment.

Human rights activists called the verdict lenient at the time, protesting the stiff penalties on defendants seen as holding Islamist views and taking a soft view of alleged crimes committed by security forces.

Journalists were forbidden from attending the hearing held Saturday at the Cairo police academy, a Reuters witness said.

State news agency MENA reported the four police officers on trial "cheered with joy" at the judge's decision. They yelled"long live justice!" after the judge read his decision, according to the website of official newspaper Al-Ahram.

Hundreds of Brotherhood supporters and members of the security forces have been killed in the upheaval since former army chief Abdel Fattah al-Sisi overthrew Mursi. Egypt's biggest political force until last year, the Brotherhood has been outlawed and driven underground.

Thousands of Islamists have been jailed in the state crackdown, along with some secular activists.

Mr Sisi will be sworn in as president on Sunday.