The CBI would take over the case from Siwan Police which said that Ranjan's killing was part of a premeditated conspiracy.
The journalist was shot dead in May at a busy market place near Station Road in Siwan district. Ranjan was the Siwan Bureau Chief of Hindi newspaper Hindustan, which is part of the Hindustan Times Media company.
The state government had constituted a Special Investigation Team (SIT) comprising two Deputy Superintendents of Police, three Inspectors and five Sub-Inspectors to trace the killers.
The Bihar government had already recommended a CBI probe into the killing.
Earlier in the day, the wife of the slain journalist Asha Ranjan urged Bihar Chief Minister Nitish Kumar to provide her justice following the release of former RJD MP Mohammad Shahabuddin from jail.
Shahabuddin, who is alleged to be involved in Ranjan's killing, was released on bail on September 10 by the Patna High Court.