The inquiry centers on a group of current and former centre-left lawmakers who are accused of accepting cash and favors from Qatar and Morocco in exchange for pushing for decisions by the European Parliament that would benefit those countries economically and politically.
Both Qatar and Morocco have denied any wrongdoing.
The Belgian police have recovered over €1.5 million in cash hidden in suitcases and jewelry boxes, and raided over 20 private residences and offices as part of the investigation.
Four people linked to the Parliament have been arrested and charged with forming a criminal organization, corruption and money laundering. Mr. Cozzolino and Mr. Tarabella could face the same charges, which carry sentences of up to 15 years in prison.
The investigation has led to the arrest of Eva Kaili, a former vice president of the assembly, who is from Greece; Francesco Giorgi, her partner and an aide to Mr. Cozzolino; Pier Antonio Panzeri, an Italian former lawmaker and Mr. Giorgi’s former employer; and Niccolò Figà-Talamanca, the head of a Brussels-based human rights charity.
Mr. Panzeri’s wife and daughter have also been arrested, in Italy, and are awaiting extradition.
Prosecutors took a big step in strengthening their case last month, when Mr. Panzeri, who is described by the Belgian authorities as the central figure in the case, struck a deal to trade information in exchange for a lighter sentence. As part of that agreement, Mr. Panzeri pledged to reveal the identity of the others who were involved.
Ms. Kaili has denied any wrongdoing through her lawyer, as has Mr. Panzeri’s wife and daughter. Mr. Giorgi has not commented.
It remains unclear what Qatar and Morocco might have been trying to accomplish. The European Parliament has the least power of the EU institutions when it comes to foreign policy, but its resolutions on human rights carry weight and can influence countries’ reputations. New legislation, such as visa waivers or trade agreements, requires the approval of lawmakers.