Eight people have been arrested in the north Iraq city of Kirkuk for allegedly planning to kill the judge who prepared the case against Saddam Hussein.
There is tight protection for the officials involved in the case
The men, arrested four days ago, had a letter from Saddam Hussein's former deputy Izzat Ibrahim al-Douri ordering the murder of Raed Juhi, police said.
Col Anwar Khader Mohammed made the announcement a day before Saddam Hussein's trial resumes in Baghdad.
Mr Juhi is one of few officials in the case to have appeared in public.
Two defence lawyers have already been killed.
Saddam Hussein and seven other top ranking officials from the ousted regime are on trial over the 1982 massacre of Shias in Dujail.
The charge says that 143 people, mostly men, were killed in the largely Shia town, some 60km (35 miles) north of Baghdad, after a failed assassination attempt against the former leader.
Saddam Hussein could face execution if found guilty.
His former deputy was reported to have died early this month, but doubts have been raised about the announcement - made by the former ruling Baath party and withdrawn from websites within 24 hours.
Douri, 63, was the second most powerful man in Saddam Hussein's Iraq and is regarded by the US as their most wanted man after the leader of al-Qaeda in Iraq, Abu Musab al-Zarqawi.