Manchester Arena bomber Salman Abedi was in the city of Dusseldorf - Germany's radical Islam hotbed - four days before killing himself and 22 innocent victims. And intelligence authorities have also discovered the ISIS disciple visited the banking metropolis of Frankfurt in 2015 - also home to a number of jihadists under the microscope of spies. German security sources said intelligence efforts have been redoubled to find if Abedi had accomplices in the country, received any explosives training or had been radicalised by radical preachers here. Four suspected extremists have been arrested in Berlin in the last 24 hours. Abedi flew to and from the city on the Rhine directly from Manchester. Dusseldorf is in the state of North Rhine-Westphalia where Berlin Christmas market killer Anis Amri, 24, worshipped at jihadist mosques shortly before he ploughed a stolen lorry into a crowd, killing 11 and wounding dozens more.And last June four ISIS terror suspects were arrested for plotting to carry out a series of bombings and shootings in Dusseldorf. They were detained after one of the conspirators turned himself in to French authorities. Focus Magazine in Germany reports today that Abedi was on no international search list monitoring the movement of suspicious Islamists and slipped with ease in and out of the country. German anti-terror experts from the police and the domestic intelligence service are now probing his potential contacts with radical German Islamists in North Rhine-Westphalia.