They added that “there were no witnesses, no identifiable patterns and no forensic leads that pointed to human involvement” in any of the cases.

The long investigation created extra workload for London officers, according to BBC News. Each case was probed individually with consultations from animal rescue groups and other experts.

While police have concluded their investigation, one of the groups involved released a statement skeptical of the results.

South Norwood Animal Rescue League said in a statement that the police’s announcement came as a “surprise.”

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“We consider that the evidence we have gathered over the last three years does indicate human involvement and there is expert opinion to back this up,” the group wrote on Facebook.

The animal advocacy group said they are seeking advice on how to move forward, and will first reach out to families affected by the feline deaths.

— With files from The Associated Press