BERLIN—A 16-year-old boy was detained Thursday in connection with a possible plan for an Islamic extremist attack on a synagogue in the German city of Hagen, authorities said. The detention took place on Yom Kippur, the holiest day in Judaism, and two years after a deadly attack in another German city on the Yom Kippur holiday. Police cordoned off the synagogue on Wednesday and a worship service planned for the evening was called off. Officials had received “very serious and concrete information” that there could be an attack on the synagogue during Yom Kippur, said Herbert Reul, the interior minister of North Rhine-Westphalia state, where Hagen is located. The tip pointed to “an Islamist-motivated threat situation,” and named the possible timing and suspect, he added. Police using sniffer dogs found no dangerous objects in or around the synagogue, Reul said. On Thursday morning, the 16-year-old, a Syrian national who lives …