The Kremlin-friendly oligarch appeared in court in Madrid on Monday as part of a separate similar case in which judges are investigating allegations he acted to bring down the value of another Spanish takeover target, digital entertainment firm Zed Worldwide.
He denied all charges in that case in a statement released after he was questioned in court.
An investigating judge with the National Court “has begun investigating a complaint” against Fridman and his Luxembourg-based investment company LetterOne “in connection with its acquisition of Dia”, according to a document from the court published Thursday.
In May, LetterOne secured majority control of the struggling supermarket chain via a hostile takeover following a bitter dispute with its previous management as the firm's share price slumped.
The judge is investigating allegations made in an anonymous complaint that LetterOne “maintained a heightened financial tension to lower the share price, until it managed to buy the company,” the court document said.
Spain's Supreme Court had in September given the National Court a mandate to investigate this case which it said could constitute the crime of “market manipulation” and could have had “serious repercussions on… the national economy” given the size of Dia's supermarket network in the country, the document added.
It cited a police report alleging that Fridman acted in a “coordinated and concerted way” through a network of “criminal associates… to create a situation of conflict… and lack of liquidity in the short term” so as to lower Dia's price and buy the firm.
In a statement, LetterOne called these allegations “totally false and defamatory”.
“The reality is Dia suffered from mismanagement and accounting irregularities were discovered, which negatively affected all shareholders, including LetterOne,” it added.
LetterOne said it was “committed” to investing 1.6 billion euros to protect jobs, suppliers and keep stores open.
Through LetterOne, Fridman also controls interests in telecoms, banking, oil and healthcare.
The tycoon, who is reportedly close to the Kremlin and was listed by Forbes this year as London's richest resident, is also one of the founders of Alfa Bank, Russia's largest privately-held lender.