Informed sources said a CBI team had reached Siwan to probe the case.
The CBI took over the case from Siwan police which had registered a case on May 13. Police had said Ranjan's killing was part of a premeditated conspiracy.
The central agency registered the case under Sections 302 (murder), 120-B (criminal conspiracy), 34 (act done by several persons in furtherance of common intention) of the Indian Penal Code and Section 27 of the Arms Act against unknown assailants on the request of the Bihar government and further notification issued by the Centre.
Ranjan was shot dead on May 13 in a busy market place near Station Road in Siwan district of Bihar.
Ranjan was the Siwan bureau chief of Hindi newspaper Hindustan, a 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.
Siwan Police had registered the case on a complaint of Ranjan's wife Asha Ranjan.
Asha urged Bihar Chief Minister Nitish Kumar on Wednesday to ensure justice to her following the release of former Rashtriya Janata Dal MP Mohammad Shahabuddin from jail.
Shahabuddin, who is alleged to be behind Ranjan's killing, was released on bail on September 10 by the Patna High Court.