New Delhi, March 25: The Prime Minister's Office (PMO) has sought the opinion of the CBI director on the merits of the Ranaghat rape-and-robbery case before taking the final call on the Bengal government's request for a probe by the country's premier investigative agency.

Sources in the department of personnel and training (DoPT) said the final decision was likely to be communicated to the state government by Thursday.

"After getting the request from the state government in the prescribed format, the PMO forwarded it to the CBI director, Anil Sinha, yesterday, seeking his opinion on the case. We want his opinion before taking the final decision, which is likely by Thursday," a senior official of the DoPT, which reports to the PMO, said.

CBI sources said the agency's brass today raised questions on the merits of the case, saying it was a "simple crime" concerning the law and order of the state and the local police were best suited for the job.

"This is not a case where our expertise is needed. The local police have the wherewithal to probe the case under the supervision of senior officials," a senior CBI officer said.

According to the norm, the DoPT normally asks for the CBI's comments on a reference received from a state government for an investigation by the agency into a case that is already being probed by the local police.

"In that case, the concerned branch head of the CBI is asked to offer his comments by looking into the documents of the state police," the CBI officer said.

Another senior CBI officer said the agency today conveyed to the DoPT its reservations about taking over the Ranaghat probe, saying the local police had the required capabilities to investigate it.

They also said the agency was overburdened with several high-profile cases, including the inter-state Saradha scam.

The DoPT official said the Prime Minister would weigh the CBI director's inputs before taking the final call.

"It seems unlikely that he will decline a CBI probe," he said.

On March 14, a gang entered the Convent of Jesus and Mary High School in Nadia's Ranaghat, allegedly raped an elderly nun and stolen cash amounting to lakhs.

In the Nadia town, a CID team today detained an expelled student of the school and his mother for questioning.

The two were picked up from their home in Don Bosco Para, around 200 metres from the school. They were first questioned at the Ranaghat sub-divisional police officer's office and later sent to the CID headquarters at Bhabani Bhavan in Calcutta.

The Class IX student was expelled last month for a Facebook post maligning a classmate. His father had allegedly threatened to sexually assault the school principal.

On March 6, the father of the student was seen shouting near the main gate of the institution along with his associates. The CID has not been able to question the father as he left for his workplace in the Gulf on March 11.

The principal had informed the local administration the same day, both over phone and in writing, saying the teachers and nuns at the school were feeling insecure.