Professor Christopher Browning, a world expert on the Nazis and the Holocaust, said that while the Germans were happy to shoot men and women, they preferred not to shoot children.
"Germans shot adults and local police shot children because that was more unpleasant," he said at the Old Bailey in London. Professor Browning said the Nazis typically recruited men in their 20s.
The prosecution alleges that Mr Sawoniuk, 77, from south London, was one of the first to volunteer and rose to the rank of Commander before fleeing with the Nazis in 1944 when the Red Army counter-attacked.
Mr Sawoniuk is alleged to have murdered 20 Jews while serving as a police officer in his home town of Domachevo, in Nazi-occupied Belarus between 1941 and 1944. He is charged on four specific counts, all of which he denies.
Yesterday Mr Sawoniuk came face to face with a former schoolfriend who is among those who accuse him of murder. Fedor Zan, 75, who alleges that he saw Mr Sawoniuk shoot 15 women in a forest clearing, stood in the dock at the Old Bailey as he was sworn in as a witness.
Next week Mr Zan will accompany the jury, which, in an unprecedented move, will travel to Belarus to visit the site of the alleged murders.
As a preliminary requirement to him accompanying the party Mr Zan swore the oath, aided by a translator, in a mixture of Ukranian and Belarussian.
John Nutting QC, for the prosecution in Britain's first war crimes trial, said that in accordance with British law, "Mr Zan has to be sworn in as a witness in this court and before this jury, so that he may be on oath when he is asked to do what he will be asked to do on Tuesday in Belarussia".
Mr Sawoniuk is said to have led "search and kill" operations to find Jews who had escaped a massacre in the town carried out by Nazi troops. They killed 2,900 in one day.
Mr Nutting has previously told the court that Mr Zan says he witnessed Mr Sawoniuk murdering 15 women days after the massacre.
Drawn to a forest clearing by the sound of women screaming, Mr Zan went to investigate and hid behind bushes.
"From a distance of 30 metres and concealed by bushes, he saw about 15 Jewish women of mixed ages with yellow patches on their clothing standing in front of an open grave," said Mr Nutting.
"The defendant was standing behind the women armed with a sub-machine gun. He ordered the women to remove their clothes, and then shot them with the weapon.
"As they died they collapsed into the grave. The witness observed this scene for some minutes. The defendant was alone," he claimed.
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