BERLIN (Reuters) - Germany will review thousands of political asylum decisions, a senior official said on Thursday, as prosecutors investigate an official suspected of approving applications by people who did not qualify.

Over one million migrants, many fleeing conflict in the Middle East, have arrived in Germany since mid-2015, raising concerns for social cohesion and security that have boosted support for the far-right Alternative for Germany, which won seats in the national parliament in September’s election.

Deputy Interior Minister Stephan Mayer told the Bundestag (lower house of parliament) that the Federal Office for Migration and Refugees (BAMF) would review 4,568 asylum rulings made between Jan. 1, 2013 and Jan. 16, 2017.

Certificates of recognition will be annulled where necessary if legally possible, said Mayer, a member of the Bavarian Christian Social Union (CSU) - sister party to Chancellor Angela Merkel’s conservative Christian Democrats (CDU).

On Friday a prosecutor said the former head of a BAMF outpost in the northern city of Bremen was suspected of wrongly granting asylum to some 1,200 people, the majority of them members of the Yazidi ethnic group.

Mayer said several people were suspected of working with the former BAMF official, including two law firms, and that all cases in which those two firms were involved would be checked.

No manipulation had been uncovered in other BAMF outposts, according to Mayer.