A Belgian judge charged four people on Sunday over allegedly receiving money and gifts from a Gulf state to influence decisions in the European Parliament, accusations that have caused consternation in Brussels.
Prosecutors searched 16 houses and seized 600,000 euros ($631,800) in Brussels on Friday as part of an investigation into money laundering and corruption.
Six people were initially detained. Four have been charged and two released, prosecutors said in a statement. They did not name any of those involved.
Prosecutors said they had suspected for months that a Gulf state was trying to influence decisions in Brussels. A source with knowledge of the case said the state was World Cup host Qatar.
A Qatari official denied accusations of possible misconduct.
"Any association of the Qatari government with the reported claims is baseless and gravely misinformed," the official said, adding that Qatar worked through institution-to-institution engagement and in full compliance with international laws.
The European Parliament said at the weekend it had suspended the powers and duties of one of its vice presidents, Greek socialist Eva Kaili, in light of the Belgian investigation.
The Greek socialist PASOK party said in a statement that it was expelling Kaili from its ranks.
It was not immediately clear if she had been charged in the case. Her office did not answer telephone calls or respond to an email requesting comment.
Prosecutors said they had also searched the home of a second EU lawmaker on Saturday, without detaining anyone. Belgian Socialist party member Marc Tarabella confirmed it was his home and that a computer and mobile phone had been taken.
"The justice system is doing its work of gathering information and investigating, which I find totally normal. I have absolutely nothing to hide and I will respond to all questions of the investigators," he said in a statement.
European Economics Commissioner Paolo Gentiloni told Italy's Rai 3 television that the case appeared to be "very serious".
"If it were confirmed that someone took money to try to influence the opinion of the European Parliament, it will really be one of the most dramatic stories of corruption in recent years," he added.
The European Parliament is due to vote this week on a proposal to extend visa-free travel to the EU for Kuwait, Qatar, Oman and Ecuador. Some lawmakers have suggested the debate and vote should be postponed.