Police said the suspects had a letter from Hussein's former deputy ordering the assassination of Judge Raed Juhi, the BBC reported.
Juhi prepared the case against Hussein and is one of the few officials in the trial to appear in public.
Hussein's trial for his role in the massacre of more than 140 people in the Shitte town of Dujail is scheduled to resume Monday after a six-week break.
However, defense attorneys, who have been joined by former U.S. Attorney General Ramsey Clark, are expected to ask for another delay.
The trial has been marked by threats and attacks. Two defense attorneys have been killed and another fled the country.
Some prosecution witnesses have faced death threats and a menacing leaflet was distributed in Dujail, the Sunday Times of London reported.
"Anyone who testifies in the court against the leader Saddam Hussein and his colleagues will receive something from us that will not please him and that goes to that person's brothers, sons, daughters, cousins and entire tribe," it read.