(Updates with order for trial, adds 4 policemen arrested)
CAIRO, Oct 22 (Reuters) - Egypt's prosecutor general on Tuesday ordered an urgent criminal trial of four police officers charged with killing 39 members of the Muslim Brotherhood after they were detained in August, the general prosecution and a security source said.
The Islamists died after teargas was fired into the back of a police van where they were being held. The four policemen have been arrested and charged with murder and unintended injury, according to the security source.
"Investigations have proven that the officers transported 45 prisoners inside a transport vehicle that is not suitable to carry more than 24 people and fired teargas inside ... (This) led to the death of 37 prisoners and the suffocation of (two others)," the general prosecution said in a statement.
The Interior Ministry said at the time the prisoners were trying to escape from the vehicle as it was transporting them to jail and that they had also taken a police officer hostage.
"The prosecution has been investigating the incident since it happened and investigated with the seven involved police officers in three private sessions," the security source said.
"Today the prosecution ordered jail for four senior officers for four days pending trial and released the three junior ones. The four were charged with neglect and killing detainees."
The state-run Al Ahram daily said on its website that public prosecutor Hisham Barakat ordered the imprisonment of the four policemen pending further investigation on suspicion of murder.
The Muslim Brotherhood has accused the military-backed Egyptian authorities of flagrant human rights abuses against the movement since the army toppled Islamist President Mohamed Mursi in July following weeks of mass unrest over his rule.
The state denies the accusations and describes the Brotherhood as terrorists bent on destabilising the Arab world's most populous country.
Hundreds of Islamists have been killed and nearly 2,000, including Mursi and other top Brotherhood leaders, have been arrested in a concerted campaign to disable Egypt's oldest Islamist movement. (Reporting and writing by Yasmine Saleh; Editing by Michael Georgy/Mark Heinrich)