Iraq's al-Sabah quoted an Iraqi police source saying security forces found letters from al-Douri on four suspects who were recently arrested.
He said the letters outlined the "need to continue with armed attacks against the country's departments, especially the security apparatus that is being formed."
The unnamed source said the letters also instructed supporters of the former Baath regime of Saddam Hussein to join the new Iraqi army and try to attain senior ranks.
U.S. forces recently accused al-Douri, one of the highest ranking former officials still at large, of leading armed attacks against their troops and Iraqi police personnel.
The U.S. offered $10 million as a reward for information leading to his capture dead or alive.
The source said the four suspects "confessed to killing a number of policemen and plotted to assassinate" Ahmad Chalabi, a member of the interim Governing Council and head of the Iraqi National Congress.