Tejashwi Yadav said the scam is not confined to Bhagalpur district and links were spread in several districts.
So far, the government has filed 10 FIRs and arrested 12 people in the case.
Kumar's decision came after a high-level meeting on the the case in which government money was allegedly siphoned off by the and some public sector banks with government officials also being named suspects.
The secretary of the accused NGO, Priya Kumar, and her husband Amit Kumar are reportedly on the run and the Bihar police has issued a lookout notice against the two.
The state government on August 18 recommended a CBI probe into the scam. These e-statements were sent to the government officials.
Preliminary investigation had found that money meant to acquire land under the Mukhya Mantri Nagar Vikas Yojna was transferred under different names to the accounts of Bhagalpur-based cooperative society Srijan Mahila Vikas Sahyog Samiti Limited.
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He said government funds were transferred to private accounts between 2005 and 2013 when BJP leader Sushil Kumar Modi was Finance Minister and Deputy Chief Minister and Nitish Kumar the Chief Minister.
"Ever since investigation began into the scam, the amount involved has been increasing day by day... it will reach Rs 15,000 crore and will be the biggest scam ever in Bihar." the former Chief Minister said. "Unmindful of how mighty a person is, nobody would be spared in the inquiry into the scam and lawful punishment would be certainly given", he told reporters in his Vidhan Sabha chamber. Mahesh Mandal's kidneys were damaged, and he had cancer, according to ANI.
After the death of Mandal, his family created a ruckus in the hospital.
Mahesh Mandal, a key-accused in the Srijan scam, died at the Jawaharlal Nehru Medical College Hospital (JLNMCH) in Bihar's Bhagalpur district late on Sunday.
Mr. Yadav's mother and former Chief Minister Rabri Devi also demanded the Chief Minister's resignation.