(CNN) -- Prosecutors have issued an indictment for 56 suspects accused of membership in an alleged ultranationalist plot to overthrow the Turkish government, Turkey's official news agency reported Tuesday.
Twelve of the suspects have been accused of leading an illegal organization, Anatolian Agency reported.
The investigation into the alleged gang, known as Ergenekon, has been under way since June 2007, when police discovered a stash of grenades and bomb-making materials in an Istanbul ghetto.
Since then, dozens of people have been arrested, including retired army generals, journalists, police officers and businessmen.
The investigation has polarized Turkish society, which has seen the military overthrow four governments in the past 50 years.
Tuesday's 1,909-page indictment is the second to be released by prosecutors since the investigation was launched.
In the first voluminous indictment, they charged suspects with plotting to attack a NATO base in Turkey as one of a series of "bloody attacks aiming at creating serious crisis, chaos, anarchy, terrorism and instability."
Critics say the investigation is a political witch hunt. They accuse Turkey's observant Muslim Prime Minister Recep Tayyip Erdogan of targeting staunch secularists.
According to Turkish media reports, retired army generals Sener Eruygur and Hursit Tolon were named in Tuesdayıs indictment.
The two former officers were arrested last year. Both have been released from jail in recent months because of health concerns.