Indian investigators are seeking to restart a probe into the Adani group for alleged overvaluation of coal imports and have asked the Supreme Court to allow them to collect evidence from Singapore, a step they say the company thwarted for years, legal documents show.

The Directorate of Revenue Intelligence since 2016 has been trying to procure transaction documents related to Adani’s dealings from Singapore authorities.

The agency suspects many of the group’s coal shipments imported from Indonesian suppliers were first billed at higher prices on paper to its Singapore unit, Adani Global Pte, and then to its Indian arms.

Adani Enterprises and its subsidiaries, led by billionaire Gautam Adani, have successfully mounted repeated legal challenges in India and Singapore to block the documents’ release, court papers show. Adani has denied wrongdoing, saying that Indian authorities assessed its coal shipments before releasing them from ports.

In an October 9 legal filing, which Reuters is reporting for the first time, the revenue intelligence agency asked India’s Supreme Court to quash a previous lower court order that allowed Adani to block authorities from collecting evidence from Singapore.

The Indian officials dispute Adani’s arguments that the agency did not follow “due process”, saying in the filing it had approval to seek information from the Southeast Asian city-state under a mutual legal assistance treaty.

“Permission for following this route was obtained from the ministry of finance as well as (the) ministry of home affairs,” the 25-page filing states, adding that the investigation “is completely in compliance”.

In a statement to Reuters, the Adani group said it had “fully co-operated” with the authorities by providing details and documents sought more than four years ago and that “no deficiency or objection” was communicated by investigators thereafter.

The Indian revenue agency did not respond to Reuters’ queries.

The effort to revive the coal probe comes amid wider regulatory scrutiny of Adani since Hindenburg Research in January accused the tycoon and his conglomerate of improper use of tax havens and stock manipulation.

Despite Adani’s denials, the short seller’s report triggered a $150 billion plunge in the group’s stocks. India’s Supreme Court is also overseeing the market regulator’s probe of Hindenburg’s allegations.

Amid the fallout and ahead of India’s 2024 election, political opponents have increased pressure on Prime Minister Narendra Modi’s administration, accusing it of favouritism toward Adani in government decisions.