Khan (53), is alleged to have stashed huge amount of black money abroad including over USD 8 billion in the Union Bank of Switzerland (UBS) in Zurich.
Khan was arrested in Mumbai on March 7 after several hours of grilling in the wake of the Supreme Court's criticism of the government over its inaction against the black money hoarders, particularly him.
The arrest of Tapuriah, a Kolkata businessman, followed soon. Both are lodged in judicial custody in Mumbai.
Khan is also under the scanner for terror links as he is suspected to have laundered money of international arms trader Adnan Khashoggi, who is alleged to be associated with terrorist outfits.
There have been allegations against Khan of money laundering, engaging in hawala operations, transferring cash and valuable assets outside the country by unlawful means with the help of foreign nationals and forgery.
He has been accused of obtaining passports from Mumbai and Patna using forged documents.
Puducherry governor Iqbal Singh has also been questioned by the ED for helping Khan in securing the travel document from Regional Passport Office in Patna through a Bihar Congress politician Amlendu Kumar Pandey, who is also under investigation. The statements of Singh and Khan are said to be part of the charge sheet.
Khan, according to ED, has reportedly confessed to stealing antiques belonging to the family of Nizams, the erstwhile rulers of Hyderabad state, from the famous Salar Jung Museum and selling them to off-shore customers for Rs 50 lakh.