A police officer stands guard at the scene of a bomb blast in Istanbul January 6, 2015. [Photo/Agencies]

Turkey's Dec.17 graft probe swirled around the inner circle of then-prime minister Tayyip Erdogan and led to the resignation of the ministers of economy, interior and urbanisation.

The commission, dominated by members of the ruling AK Party, voted on Monday not to send the four ex-ministers for trial, a decision that the opposition decried as a cover-up of one of Turkey's biggest ever corruption scandals.

DHKP-C (Revolutionary People's Liberation Party-Front) also claimed responsibility for a grenade attack on police near the Turkish prime minister's office in Istanbul last week.

The group, which had since then pledged further attacks, was also behind a suicide bombing at the U.S. Embassy last year as well as attacks on police stations.