Abdelhamid Abaaoud, the Belgian militant suspected of having orchestrated the Paris terrorist attacks was killed in a police raid in the northern Paris suburb of St.-Denis early Wednesday, the French authorities announced on Thursday. The death of the plot’s alleged ringleader ended one chapter of the intense criminal investigation that began on Friday night, when three teams of terrorists, in a series of closely coordinated raids, killed 129 people. But many questions remained unanswered: how Mr. Abbaoud planned and organized the attacks; whether the Islamic State is planning additional attacks outside of its stronghold in Syria and Iraq; and the identities of several of the attackers. Also on Thursday, the Belgian police conducted their own sweep in Brussels on Thursday. Seven raids are being conducted in Brussels in relation to Bilal Hadfi, one of the dead Paris attackers, a spokesman for the Belgian federal prosecutor said, adding that the houses of Mr. Hadfi’s friends and relatives were being searched. One person has been detained for questioning.