Belgian investigators searched offices at the European Parliament in Brussels on Monday, in a probe into alleged bribes from World Cup host Qatar.
It was the 20th search carried out in Belgium since Friday and the parliament president, Roberta Metsola, warned MEPs that “democracy is under attack”.
Belgian police have arrested four people, among them European Parliament Vice-President Eva Kaili, a Greek MEP.
Qatar has denied any wrongdoing. MEPs will debate the scandal on Tuesday.
Belgian prosecutors said cash worth about €600,000 ($632,000; £515,000) was found at the home of one suspect, €150,000 at the flat of an MEP and several hundred thousand euros in a suitcase in a Brussels hotel room. Computers and mobile phones were also taken.
Since Friday, the IT resources of 10 parliamentary employees have been “frozen” to prevent the disappearance of data necessary for the investigation.
The four suspects arrested by Belgian police have been charged with “participation in a criminal organization, money laundering and corruption”, prosecutors said in a statement on Sunday.
Kaili – an MEP for eight years – was suspended from her duties as one of 14 vice presidents by parliament president Metsola.
She has also been suspended from the parliament’s Socialists and Democrats Group and expelled from the Greek center-left Pasok party.