Senior TMC leader Abhishek Banerjee's wife Rujira appeared before the Enforcement Directorate (ED) in Kolkata on Thursday for questioning in the coal pilferage case, officials said.
A panel of five officers, including those from New Delhi, was interrogating Rujira, they said.
According to an ED source, the investigators have prepared a three-page questionnaire for her.
Rujira was stopped at the Kolkata airport on Monday when she was going to UAE with her two children. She was then served the summons for appearing before the agency at 11 am on June 8.
However, she reached the CGO Complex around 12.40 pm with her lawyer.
Abhishek, the nephew of Chief Minister Mamata Banerjee, alleged the "harassment" of his family was aimed at stopping his outreach campaign ahead of the panchayat elections in West Bengal.
Tight security arrangements were made around the CGO Complex to prevent any untoward incident, a senior police officer said.
Both CBI and the ED earlier questioned Rujira in the case.
The CBI was probing the scam over illegally-mined coal from the abandoned mines of Eastern Coalfields, estimated to be worth thousands of crores of rupees, officials said.
They said the crime proceeds were transacted through the Hawala route, which the ED was probing.
The Trinamool Congress (TMC) alleged the ED's action to be a political vendetta by the BJP.
"Whenever Abhishek Banerjee was summoned, he cooperated. But the way his family is being harassed is shameful. This is nothing but political vendetta by the BJP," TMC's state general secretary Kunal Ghosh said.
"The BJP is afraid of the TMC's 'Nabojowar' campaign ahead of the panchayat polls, and also wants to divert the attention from the train accident," he claimed.
The state BJP said the CBI and ED were independent agencies, devoid of any political influence.
"The BJP has nothing to do with the investigation. The allegations are baseless, and if they have any complaints, let them move court," state BJP president Sukanta Majumdar said.