BERLIN—The German city of Wuerzburg on Sunday held a memorial service for the victims of a brutal knife attack that left three women dead, while questions about the possible motive of the assailant remained unanswered. Friday’s assault in and outside a store in the center of the Bavarian city left another five people seriously injured. The suspect, a 24-year-old Somali man, was shot in the leg by police and arrested after people surrounded him and tried to hold him at bay with chairs and sticks. Investigators were still working Sunday to figure out what was behind the attack by what they believe was a lone assailant. They are assessing his mental health and trying to determine whether he was radicalized as an Islamic extremist. It was unclear whether he deliberately targeted women. “We are all affected, because it could have happened to anyone,” Wuerzburg’s Catholic bishop, Franz Jung, said at …