TOKYO (Kyodo) -- A Russia-linked hacking group e-mailed multiple executives of Japanese publisher Kadokawa Corp. that it had received $2.98 million in cryptocurrency from the firm after it was hit by a massive cyberattack in June, a company source said Thursday.

An investigation by security firm Unknown Technologies Inc. commissioned by Kyodo News found online records of a transaction worth $2.98 million made the same month. The hacking group, purporting to be the Russia-linked BlackSuit ransomware gang, ultimately leaked the stolen information.

The hackers had claimed the gang had stolen and encoded data including personal and financial information amounting to 1.5 terabytes.

The finding is expected to spark a debate on whether the alleged payment was an appropriate response to the attack.

On June 8, servers located in Kadokawa group's data center experienced a significant cyberattack, including ransomware, targeting major video streaming website niconico and related services operated by the publishing firm.

The company confirmed later that month that the personal information of all employees of Dwango Co., its subsidiary operating niconico, had been leaked.

Kadokawa declined to comment on whether it made any payment to the hacking group, citing ongoing police investigations.

The group previously asserted it had not received any such money. It has not responded to interview requests since.

"The hackers tried to apply pressure on the top management by informing them of the process as negotiations stalled," the Kadokawa source surmised.

Company executives received emails on June 8 and 17, which included interactions on June 13 between Shigetaka Kurita, the chief operating officer of Dwango, and the hacking group.

In the interactions on June 13, the hackers demanded Kurita pay $8.25 million, while Kurita had replied that he had taken on a risk by negotiating with the hackers without informing the board of directors, and that he was unable to pay more than $3 million due to strict compliance measures following bribery scandals concerning the Tokyo Olympics, according to the source.

The hackers told Kurita they would only agree to delete the stolen data if it received $2.98 million within 48 hours, the source said.

Unknown Technologies confirmed records that around 44 Bitcoins worth approximately $2.98 million at the time had been sent to a cryptocurrency account on June 13.

"Negotiations should not be made so casually as there are many hackers that don't keep promises," an official of the security firm said.