No trace of the substance was found at the scene of the stabbings in Southport and police said the poison they did discover presented a low risk to the public.
Dr Renu Bindra, senior medical advisor at the UK Health Security Agency, said: “There has been no evidence of ricin poisoning in anyone involved or connected to this incident.”
The teenager has already been charged with the murders of Bebe King, six, Elsie Dot Stancombe, seven, and Alice da Silva Aguiar, nine.
He is also charged with ten counts of attempted murder and possession of a knife.
A trial is expected to begin in January.
