Interior ministry spokesman Adil Daham made the announcement to reporters on December 19, while state-run television aired what it said were confessions by alleged terrorists linked to al-Hashemi.
The move comes a day after the last U.S. troops left Iraq and ended a war lasting nearly nine years.
Al-Hashemi is one of the leaders of the Sunni-backed secular bloc Iraqiya, which has just suspended its participation in parliament to protest the control of key posts by Shi'ite Prime Minister Nuri al-Maliki.
Iraqiya, headed by former premier Ayad Allawi, narrowly won the most seats in last year's parliamentary election, but al-Maliki kept the premier's post after gathering key support from Shi'ite parties.
Al-Maliki has effectively controlled the interior and defense ministries for more than a year, while disputes between Sunni and Shi'ite politicians have delayed the appointment of permanent ministers.