From "Debrett's Kings and Queens of Britain" by David Williamson, ISBN 0-86350-101-X, p. 43:
Succeeded by the son of a concubine.
In 933 he extended the rule of the family as far west as Couesnon.
135A pagan reaction swept through Normandy after the murder of William Longsword in 942.
135William had a reputation as a friend of the Church. There are charters of three religious houses that he founded.
135Weis' "Ancestral Roots . . ." (121E:19): "WILLIAM I, "Longsword", b. ca. 891, prob. Rouen, ca. 927 succ. to Duchy of Normandy, ca. 930 the Bretons rebelled, he subdued them, taking Brittany, the Channel Islands, the Contentin, and the Averanchin; killed in treacherous ambush 17 Dec. 942 by servants of THEOBALD OF BLOIS and ARNULF OF FLANDERS 17 dec 942. ... He m. (1) (Danish wife) SPROTA, a Breton; m. (2) LUITGARDE DE VERMANDOIS ..., n.i.,
killed 942 by LOUIS, s. of CHARLES III "the Simple:, dau. HERBERT II, Count of Vermandois and Troyes. " Also mentioned (49:19). Stuart's "Royalty For Commoners" (166:34). It is interesting to note that THEOBALD later married William's widow, LUITGARDE and that ARNULF married LUITGARDE's sister, ALIX. Per Rosamond McKitterick's, "The Frankish Kingdoms under the Carolingians 751-987", London & NY (Longman) 1983, p 238: "William Longsword, ROLLO's son and successor, was the principal architect of Normandy's development. He married the daughter of
HERBERT OF VERMANDOIS but his children were born of a Breton concubine. William had supported LOUIS IV and attempted to establish his protection over Brittany, gaining at least the Cotentin and Avranchin in 933. There remains some doubt about William's claim to the whole Breton peninsula. When in 942 William was murdered at the instigation of COUNT ARNULF OF FLANDERS, his son Richard, still a minor, succeeded him."
ROLLO died in 927, and was succeeded by his son William "Long Sword" born of his union 'more danico' with POPPA, daughter of COUNT BERENGER; he showed some attachment to the Scandinavian language, for he sent his son William to Bayeux to learn Norse. The first two dukes also displayed a certain fidelity to the Carolingian dynasty of France, and in 936 William "Long-Sword" did homage to LOUIS IV d'Outremer. He died on 17th of December 942, assassinated by the COUNT OF FLANDERS.