Singapore-based cryptocurrency organization Foundation said Tuesday it has stopped tracking stolen NEM virtual currency, as it had since a massive hack of the Coincheck exchange in January.

The foundation's President Lon Wong said in a statement posted on Twitter that since Sunday it has disabled the tracking mosaic that was put into place to monitor movements of the stolen cryptocurrency, without revealing a reason.

"This effort was effective at reducing the hacker's ability to liquidate stolen XEM and provided law enforcement with actionable information," the statement said. XEM is a unit of NEM.

"We don't plan to release further details due to the sensitive nature of this investigation," it said.

After a total of about ¥58 billion ($545 million) worth of NEM was stolen from the Tokyo-based Coincheck bourse on Jan. 26, the foundation tracked stolen coins by marking accounts used for illicit transfers and urged virtual currency exchanges around the world not to process NEM payments from the marked accounts.

But NEM with a market price of ¥29 billion at the time of the theft are suspected to have been converted into other cryptocurrencies through a site on the dark web, which can only be accessed with special anonymity software

On March 12, Coincheck announced the initiation of reimbursements for affected customers. It has promised to compensate around 260,000 holders of the stolen NEM to the tune of ¥46.6 billion.