The CBI’s probe has so far not found any evidence to suggest that the young RG Kar doctor was gang-raped and has identified the Kolkata Police civic volunteer arrested a day after the August 9 crime as the lone suspect, sources in the agency said on Friday about an investigation that they claimed was in the “final stages”.

The central agency, the sources said, has sent a medical report that includes DNA evidence related to the accused Sanjay Roy to experts at AIIMS, New Delhi, and is awaiting the doctors’ final opinion.

“We are awaiting the opinion of the AIIMS experts on the DNA and forensic reports which will help us reach a conclusion and answer all the questions related to the rape and murder of the 31-year-old postgraduate trainee. The available evidence so far only implicates the prime accused, Sanjay Roy, as the probe is now in its final stages,” a CBI officer told The Telegraph.

He added: “We have analysed electronic evidence like CCTV footage and the (mobile phone) tower location of Sanjay Roy to establish the time when he was present at the spot. All the evidence corroborates one another to suggest his involvement.”

The CBI sources questioned the police’s initial probe, dubbing it “very unfortunate”, and suggested the possibility of a “bid to destroy evidence”.

According to the officer, the final opinion of the AIIMS doctors on the DNA evidence, including the viscous fluid collected from the victim’s genitalia, would help conclude whether there was a lone assailant or multiple ones.

The Supreme Court is scheduled to hear on Monday the progress of the CBI probe into the brutality that has stirred an unprecedented upheaval and cries for justice in Bengal and beyond.

The protesting junior doctors and everyone else are awaiting clarity on several key facets of the crime, including the possibility of the involvement of more than one person in the rape and murder. Withering questions have been raised on the role of the police and civil administration and the political dispensation in the aftermath of the crime.

So far, the CBI has recorded over 100 statements and conducted 10 polygraph tests, besides arresting Sandip Ghosh, the former principal of RG Kar Medical College and Hospital on whose watch the crime took place, on the charge of financial irregularities.

Another CBI officer said: “So far there is no evidence to suggest the involvement of any other person (barring Roy). Our officers are preparing a watertight chargesheet against him.”

Calcutta High Court handed over the probe to the CBI on August 13. There has been no arrest in the rape and murder case since the central agency took over. Subsequently, the Supreme Court took suomotu cognisance of the crime and the matter is being heard by a three-judge bench headed by Chief Justice D.Y. Chandrachud.

“The agency will submit a status report next week before the apex court on the probe so far,” one of the CBI officers said.

The CBI sources termed “very unfortunate” the way Kolkata police and the civil administration handled the situation after the horrific incident. “We are also looking into the allegations of the doctor’s parents that some police officers offered them money immediately after they returned from the hospital on the day of the incident,” a CBI officer said.

Questioning the motive behind the police’s “lackadaisical approach” in not securing the crime scene and the delay in registering the FIR, the agency officer said “the police’s intent clearly suggests that there was an attempt to destroy evidence”.

He, however, added that the multiple claims and allegations of conspiracy appeared unsubstantiated at the moment.