In 1944, Jankowski joined the Polish underground resistance and continued his education through clandestine classes.
After the war, he studied law at the Jagiellonian University in Kraków, southern Poland, earning his degree in 1951.
In 1967, Jankowski joined a commission tasked with investigating Nazi crimes in the Kielce region. The commission gathered evidence and initiated investigations against suspected perpetrators.
Jankowski coordinated nationwide research into the suffering of Polish villages under German occupation and led complex proceedings concerning officials of the Third Reich, according to Poland’s Institute of National Remembrance (IPN).
His work helped establish the scale of crimes committed against rural communities, the IPN said.
In 1973, he helped secure a death sentence against Albert Schuster, a German official known as the "Butcher of Łysogóry." The sentence was carried out, according to the IPN.
Jankowski also authored scholarly publications documenting German crimes in the Kielce region and organised commemorative events honouring Polish victims of the occupation.
One of the most significant results of his efforts was the establishment of the Museum of the Martyrdom of the Polish Countryside in the south-central village of Michniów, a memorial dedicated to victims of village pacifications. The site was officially opened in 2022.
Jankowski died on February 9. Funeral ceremonies are scheduled for Friday in Warsaw. He will be buried at the city's historic Powązki Cemetery.