Siemowit I of Masovia

Siemowit I of Masovia (d. 23 June 1262), was a Polish prince member of the House of Piast, Duke of Czersk during 1247–1248, Duke of Masovia (except Dobrzyń) during 1248–1262, ruler over Sieradz during 1259–1260. He was the sixth son of Konrad I of Masovia and his wife Agafia of Rus. Life Siemowit appears clearly in documents only in 1239, when at the behest of his father, he probably killed the canon of Płock Jan Czapla; however, and despite his personal involvement in this matter, current historiography completely blames his father for his action.