The investigation was opened on July 14, before President Kais Saied used emergency powers to dismiss the prime minister and freeze parliament in a move that both those parties have branded as a coup.
Though the investigation does not appear linked to Saied's actions, and Tunisia's judiciary spoke out on Monday to insist it remain independent, it puts more pressure on the president's main opponents, Reuters reported.
The moderate Ennahda and media mogul Nabil Karoui's Heart of Tunisia are the two largest parties in the deeply fragmented parliament elected in September 2019. The other party under investigation is the smaller Ayich Tounes.
Ennahda leader Rached Ghannouchi, the parliament speaker, and Karoui, were opponents of Saied in a separate presidential election that took place over two rounds in September and October of 2019.
Karoui, who owns a big private television station, also faces a long-running investigation into other accusations of financial offences that led to his remand in custody during much of the 2019 election campaign and again this year.
Saied -- an independent -- campaigned in 2019 as a new broom against what he painted as a corrupt, stagnant political elite focused on its own narrow interests and responsible for a decline in Tunisian living standards after the 2011 revolution.