Belgian investigators on Monday searched offices of the European Parliament as part of a probe into alleged corruption that has seen a vice president arrested, prosecutors said.
Belgium's federal prosecutor's office said the purpose of the search at the premises in Brussels was to seize computer data from 10 parliamentary staff.
The latest search is the 20th to have been carried out in Belgium since Friday into alleged bribery by World Cup host Qatar to burnish its image at the parliament.
A European Parliament vice president, Eva Kaili, was among four suspects arrested and charged with corruption by Belgian prosecutors after investigators found "bags of cash" at her home.
Kaili, who has spoken publicly in support of Qatar's recent labor reforms, was one of four suspects to have been charged and detained.
Two more have been released and the house of at least one more MEP has been searched by investigators.
Kaili and the other suspects that have been charged will appear at a pre-trial chamber in Brussels on Wednesday, prosecutors said.
The scandal erupted during the 2022 football World Cup, an event Qatar had hoped would boost its reputation but which has been dogged by allegations of mistreatment of the migrant workers who built the host's new stadiums.
Belgian prosecutors said they had suspected for months that a Gulf state was trying to buy influence in Brussels.
A source with knowledge of the case said the state was Qatar. A Qatari official denied at the weekend accusations of possible misconduct.
"Any association of the Qatari government with the reported claims is baseless and gravely misinformed," the official said.
The investigation comes as World Cup host Qatar is in the global spotlight, amid criticism of its human rights record, including its treatment of migrant workers.
The bribery claims have rocked the EU's legislature and sparked calls for the bloc's institutions to be put under the microscope to root out foreign influence.
"There is a process ongoing. Certainly the news is very worrisome -- very, very worrisome," EU foreign policy chief Josep Borrell said.
Borrell said no officials from the bloc's diplomatic service or overseas missions were implicated in the allegations.
"There is a police and judiciary actions. We have to follow these actions," Borrell said, adding he could not go beyond the "judiciary statements".
"This is very, very, very grave accusations," he said.
Irish Foreign Minister Simon Coveney said the allegations were "damaging and we need to get to the bottom of it."
And his German counterpart Annalena Baerbock warned "this is also precisely about Europe's credibility."
The arrests followed raids in Brussels on Friday which prosecutors said turned up 600,000 euros ($630,000) in cash.
Police also seized computers and mobile phones.
Kaili has been stripped of her responsibilities as a vice president of the parliament, notably that of representing the European Parliament president in the Middle East.
She remains an MEP and would normally enjoy immunity from criminal prosecution.
But there is an exception in cases where a suspect is caught red-handed in the act of committing an alleged crime.
This report was prepared using data from Reuters and Agence France-Presse.