The citizenship of Mani Kumar Subba, controversial Congress parliamentarian from Assam, on Friday came under 'serious' scrutiny before the Supreme Court, which pulled up the Centre and the Central Bureau of Investigation for failing to arrive at a conclusive finding on his nationality.

The court was anguished that the CBI gave pre-dominance to the 'highly suspicious' documents and affidavits filed by the Lok Sabha member from Tezpur for contesting the elections while investigating the issue and did not put the onus on him to prove his nationality.

A bench comprising Chief Justice K G Balakrishnan and Justice D K Jain took strong exception that the agency did not bother to place on record documents relating to Subba's birth certificate and school certificate. The bench was perturbed that the CBI in its report only said that there was nothing to establish that he was a Nepalese citizen.

"Prima facie there are serious allegations and we have to take serious view," the bench said, adding that the CBI was not proceeding in right direction.

The court was hearing a public interest litigation filed by Noida-based Birendra Nath Singh, who had alleged that the lottery baron from Assam was a Nepalese citizen and had escaped to India after a criminal case of murder was registered against him in the early 1970s.

The bench was not impressed with the arguments of Additional Solicitor General Gopal Subramanian and senior advocate Abishekh Manu Singhvi, appearing for the Congress parliamentarian that the CBI had gone into the depth of the matter with the Nepalese government and even took the help of the Interpol, and that it was only a witch-hunt aimed at maligning Subba.

"First find out what is in India then go to Nepal," the bench told the ASG and wanted to know whether the CBI had questioned Subba.

"Subba has to prove nationality. He has to prove his credentials," the bench said when the ASG tried to convince the court that the allegations against the parliamentarian were not true as he was born in a village in Darjeeling district in West Bengal from where his family migrated to Sikkim and then to Assam.

Observing that there was 'serious conflict' on Subba's citizenship, the bench said: "Had it been the case of ordinary citizen, it would not have a contradictory version."

Maintaining that Subba was holding a responsible position, the bench said that he should show where he was born as everything on his citizenship was 'highly suspicious.'

"The CBI is unable to show that he is a citizen of this country," it said, expressing its displeasure that the agency did not even have documents like Subba's birth certificate, school certificate or any evidence where the parliamentarian studied.

The court concluded Friday's hearing by granting six weeks to the CBI and Subba to file before it additional documents on the contentious issue of his citizenship. The petition had alleged that Subba has changed his name from Limbo after fleeing from Nepal, which was refuted by Singhvi who claimed that there was no evidence to suggest that Subba was the same Limbo.