Here are the 10 latest developments of the story
Mr Modi has accused Tej Pratap Yadav, 28, of getting the Patna zoo to source soil for earth-filling work from the under-construction site for a mall that is linked to the Environment and Forest Minister and his siblings.
The BJP leader had alleged that Zoo authorities paid Rs 90 lakh for the soil. But government officials told NDTV that the zoological park had bought soil worth Rs 44 lakh over the last three months.
Zoo officials say the soil wasn't bought from the developer tasked to build what was promised as Patna's biggest mall. They have, however, conceded that the purchases were made without seeking bids.
"There was complete nexus and at the behest of Tej Pratap Yadav, this soil was purchased," Mr Modi alleged.
Lalu Yadav has denied the charges. "There must be some record about money being transferred in the name of my family members. I am ready to face any probe to prove my family's innocence into the matter," he said.
The BJP had lost the 2015 elections that gave Nitish Kumar his second term. This time, however, the Janata Dal (U) leader had to join hands with Lalu Yadav's Rashtriya Janata Dal. Tej Pratap Yadav became the minister for Forests while his other son is the state's Deputy Chief Minister.
The JD(U) has backed the Yadav family and pitched its spokesman Sanjay Singh to defend its Forest Minister. Mr Singh hit out at the Mr Modi, a former Deputy Chief Minister for making "baseless statements".
This isn't the first time the Yadav family has been accused of corruption. The senior Yadav had lost his Lok Sabha seat in 2013 when he was convicted in the Fodder Scam detected in the mid-eighties during an audit by the top auditor panel, CAG.
Mr Modi had a key role to play; he had pursued court cases against Lalu Yadav for the scam that was investigated by the CBI in a court-monitored probe.
Over 50 cases were registered for embezzlement of funds on the pretext of buying fodder for cattle, among others.
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