Officers' Association demanded a CBI inquiry to ensure an independent and fair probe.
"The SIT has arrested BSSC chairman Sudhir Kumar and four others, including his close kins, in connection with the paper leak matter," Patna Zonal Inspector General of Police N H Khan told PTI.
"The SIT has arrested Kumar on the basis of concrete evidence of his role in the entire matter of paper leak," Khan said.
Kumar has been arrested from Jharkhand's Hazaribagh district, a senior police source said, adding of the four others arrested along with Kumar, two are his nephews.
The IAS Officers' Association expressed deep anger and anguish over Kumar's arrest and demanded a CBI inquiry to ensure an independent and fair probe into the alleged scam.
It also said that the Association would take up the matter with Chief Minister Nitish Kumar.
The association in a press release also termed the arrest as "completely unwarranted" and demanded his immediate release, saying Kumar is an honest and sincere officer and was cooperating in the investigation.
"Sudhir Kumar has all along been cooperating with the investigation and, therefore, the arrest is absolutely unwarranted. The Association strongly demands that this matter be transferred to the CBI forthwith in the interest of justice, independence and fair play," the Association's Bihar Branch Secretary and IAS Officer Vivek Singh said.
The release said that the association has "decided to bear all legal and other expenses on his behalf".
The association's treasurer Dipak Kumar Singh said the SIT has apprised the Association of the sequence of events leading to the late-night arrest of the officer.
The association met and apprised the state Chief Secretary Anjani Kumar Singh of its concerns besides seeking immediate release of the officer, Singh said.
The Bihar government had on February 8 last cancelled the BSSC examination held to fill up the posts of clerks in state government jobs in the wake of paper leak.
Chief Minister Nitish Kumar had announced the cancellation of BSSC examination on the basis of preliminary report of state police chief P K Thakur and recommendation of the Chief Secretary.
The leaked paper of BSSC examination for clerk jobs became online before start of the examination on February 5.
Earlier, SIT headed by Patna Senior Superintendent of Police Manu Maharaj had arrested Commission Secretary Parmeshwar Ram and one other staff of the Commission in the case.
Ram's arrest came after intensive questioning by the SIT after taking him into custody on February 8.
The latest examination irregularities in the BSSC follows the infamous toppers' scam in the state plus two examinations in 2016.
Media reports suggested huge amounts ranging from Rs 2 to 5 lakh was collected from candidates for the BSSC exam for clerical grade job.
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