New Delhi: Himanta Biswa Sarma and Mukul Roy, who had defected to the BJP from the Congress and the Trinamool Congress respectively, were among those who were questioned by the CBI in the Saradha scam but probe into their alleged involvement has not reached any conclusive end yet.

Amid the ongoing tussle between the West Bengal government and the Centre over probe into the Saradha scam in which the CBI wants to question Kolkata police commissioner Rajeev Kumar, Assam Congress has accused the CBI of going slow against Sarma, who had switched sides and joined BJP.

The CBI did not respond to questions on the dates the two have been questioned or have been searched in these cases.

"The CBI is yet to take any action against Sarma and investigations against him have stopped after he joined the BJP," Assam Pradesh Congress spokesman and former minister Pradyut Bordoloi said.

West Bengal Chief Minister Mamata Banerjee had alleged on Sunday night that Sarma, Assam's health and finance minister now, was involved in the chit fund scam, he said.

"He (Sarma) was investigated for his involvement in the scam while he was in Congress but after he joined the BJP, it (investigation) stopped," Banerjee said.

Sarma, who was a cabinet minister in the Congress government, was questioned by the CBI on November 26, 2014 in connection with its probe in Saradha chit fund scam.

He was asked to appear before the Special Investigation Team headed by the then Joint Director Rajiv Singh at CGO Complex in Kolkata.

He was questioned for his purported proximity with Saradha group chief Sudipta Sen, sources said.

A powerful minister in Tarun Gogoi Cabinet since 2001, Sarma was dropped by the chief minister after he spearheaded a campaign to dislodge him.

The agency had also searched his premises in connection with the scam probe on August 28, 2014 along with former minister Anjan Dutta, who was recently questioned as well, and singer Sadananda Gogoi, who was arrested by CBI.

Sarma joined the BJP on August 28, 2015 ahead of assembly elections in the state that year. After this, no coercive action has been taken against him by the CBI, the sources said.

"The frivolous and baseless campaign by @Mamataofficial Didi against me is very painful. I am not fortunate enough like your police commissioner. I have joined the investigations and offered my full cooperation as a witness. And all this happened much much before I joined @BJP4India," Sarma tweeted Tuesday.

The BJP had mentioned Sarma's "involvement" in the Louis Berger case in the party's manifesto during the 2014 Lok Sabha but "once he joined BJP, the party became silent and trained their guns at West Bengal", Bordoloi said.

Roy, once a trusted lieutenant of Mamata Banerjee and general secretary of the party, was also questioned by the CBI in connection with Saradha scam.

The former railway minister was questioned on January 30, 2015 by the CBI in connection with the Saradha scam in Kolkata. But he was not named as an accused in the FIR or charge sheet so far.

Two years later on April 16, 2017 he was booked in Narada sting case by the CBI along with several other TMC leaders like Madan Mitra, the sources said.

Roy, then a TMC MP, was summoned by the CBI to appear before the agency for questioning on September 11, 2017 with regard to his alleged involvement in the "scam", a sting operation allegedly involving various leaders of the ruling party in West Bengal.

The sting operation was conducted by Matthew Samuels, the CEO of Narada News.

The tapes show Samuels, posing as a businessman, purportedly offering money to the TMC leaders.

Two months after questioning, Roy switched sides, resigned from Rajya Sabha membership and joined the BJP on November 3, 2017.

No apparent coercive action has been taken against the two after they switched sides, the sources said.

"The investigation into the Saradha chit fund case has been going on since 2014. In 2015, the CBI called me for questioning and I was questioned by them for eight hours. After that, I was with TMC for two years. I conducted the 2016 assembly election for the party," Roy told reporters Monday.

He said the CBI is an independent agency which has called people from various parties for questioning at one point of time or the other.

"So, those who don't have anything to hide will comply. It had nothing against me. CBI is investigating according to the orders of the Supreme Court and Mamata should let it work," said Roy.