State "omissions" meant that Co Armagh solicitor Rosemary Nelson was more at risk and vulnerable to the loyalist paramilitaries who eventually murdered her.
The inquiry established to examine allegations of collusion concluded there was no evidence of any act by the RUC, British army or Northern Ireland Office (NIO).
However the inquiry team did find that some RUC members "publicly abused and assaulted Rosemary Nelson... having the effect of legitimising her as a target".
The inquiry, which cost £46.5 million, also concluded that there was "some leakage of intelligence which we believe found its way outside the RUC".
The findings, included in the 505-page report released in Belfast today, admit that such a leakage "increased the danger to Rosemary Nelson's life.
The 40-year-old mother of three died after a loyalist booby trap device exploded under her car on March 15th 1999.
The inquiry members said: "We cannot exclude the possibility of a rogue member or members of the RUC or army in some way assisting the murders to target Rosemary Nelson".
Detailing a series of findings, the report cites "omissions" by state agencies, the report concludes that the RUC "negligently failed to intervene to prevent their officers from uttering abuse and threats to defence solicitors, including Rosemary Nelson".
It also cites failures to provide special attention for Mrs Nelson, failing to analyse or evaluate intelligence relevant to her and failing also to warn her of her vulnerability or offer her security advice.
The NIO was also at fault, the inquiry team finds. It failed to press the RUC for "full replies to their questions concerning Mrs Nelson's security. The report said the NIO should also have "proactively questioned" the RUC as to what factors were considered in producing a threat assessment.
The NIO further failed Mrs Nelson by dealing "in a mechanistic way" with correspondence from non-governmental organisations raising concerns about Mrs Nelson's safety.
"The combined effect of these omissions by the RUC and NIO was that the state failed to take reasonable and proportionate steps to safeguard the life of Rosemary Nelson," the inquiry reported.
"If Rosemary Nelson had been given advice about her safety and offered security measures, then assuming that she had accepted such advice and security measures, the risk to her life and her vulnerability would have been reduced."