The piles of human remains were discovered in April 2023 in Shakahola forest, a vast bushland that lies inland from the Indian Ocean town of Malindi.
Kenyan authorities are due to start releasing the exhumed bodies to relatives for burial next week after the DNA profiling was delayed by lack of reagents and equipment.
The Kenya National Commission on Human Rights, KNCHR, a state-backed body, criticized security officers in Malindi for "gross abdication of duty and negligence."
"They not only failed to be proactive in collecting and acting on intelligence to forestall the Shakahola massacre but also unjustifiably failed to act on credible and actionable reports," KNCHR chairwoman Roseline Odede said.
Self-professed pastor Paul Nthenge Mackenzie is alleged to have incited his followers to starve to death for them to "meet Jesus."
While starvation appears to be the main cause of death, autopsies carried out by the government found that some victims — including children — had been strangled, beaten, or suffocated.
Between April and October last year, a total of 429 bodies were exhumed from shallow graves, while 67 adults and 25 children were rescued, according to government records.
Cult leader and self professed Pastor Paul Nthenge Mackenzie, arrives at the Malindi Law Courts under tight security, on December 1, 2023 for the sentencing of his 2019 case.
KNCHR said "many followers faced a slow excruciating death, mainly through starvation."
The rights group said the radicalization of Mackenzie, a former taxi driver, had been raised at a court users' meeting in November 2019 but was ignored.
A former follower of Mackenzie had "desperately" tried to sound the alarm, but her warnings were also dismissed, it added.
"Instead of investigating the veracity of the issues raised, the lady was intimidated after being accused of making baseless accusations," Odede said.
"The commission regrets that no known sanctions were taken against those officers who abdicated their duty to protect hundreds of persons, including children who are either missing, dead or deeply traumatized," KNCHR said.
Mackenzie has been in detention since handing himself in to the police in April and investigations continue. He has been charged with murder, manslaughter, terrorism, and child abuse.
A Senate commission of inquiry reported in October that Mackenzie had faced charges back in 2017 for his extreme preaching but "the criminal justice system failed to deter the heinous activities."