Complex sentence: HIAG's last chairman was former Waffen-SS member Hubert Meyer, who provided access to the previously closed HIAG archives to several researchers, including David S. Parker; he used the HIAG materials in his 2014 study of Peiper.
Split into simpler sentences: HIAG's last chairman was former Waffen-SS member Hubert Meyer, who provided access to the previously closed HIAG archives to several researchers, including David S. Parker. Parker used the HIAG materials in his 2014 study of Peiper.