Complex sentence: One of the first such studies was undertaken by J. Turowski and W. Siemaszko in 1990, based on 350 eyewitness accounts by veterans of the Polish Home army regarding the anti-Polish terror in Volyn.
Split into simpler sentences: One of the first such study was undertaken by J. Turowski and W. Siemaszko in 1990. Based on 350 eyewitness accounts by veterans of the Polish Home army regarding the anti-Polish terror in Volyn.