"We've been too naïve ... for far too long," a justice ministry spokesperson said, referring to what he called clandestine operations by foreign powers in Belgium.
The other three suspects arrested and charged last week were meanwhile questioned, as planned, on Wednesday by a three-judge panel.
Niccolo Figa-Talamanca, the secretary-general of a rule of law campaign group, will leave jail but wear an electronic ankle tag.
The European Parliament on Tuesday voted to strip Kaili, a 44-year old Greek Socialist MEP, of her vice presidency role.
Although no state was publicly named by prosecutors, a source with knowledge of the case said it was Qatar.