The trial recruited 128 healthy volunteers aged 18–55, who were paid €1,900 (US$2,060) each.
Ninety people received different doses of the drug, and the remainder a placebo.
The trial had tested escalating single doses of the drug without observing any serious adverse side effects.
The six participants who fell ill were the first to receive repeat higher doses over the course of several days.
The first participant to fall ill experienced adverse symptoms on 10 January and died on 17 January.
Biotrial halted the trial on 11 January; the other five affected people were hospitalized in the days that followed.
One of these patients has since been discharged, and the condition of the other four is judged to be serious but stable.
Authorities are contacting the 84 other people who received the drug at lower doses to arrange medical check-ups; none of the 18 given neurological check-ups over the weekend showed any of the symptoms seen in the hospitalized people.
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