Venezuelan authorities said Friday they were opening a new investigation into Rafael Ramirez, the former head of state oil company PDVSA, who is already accused of graft.

The matter, announced by chief prosecutor Tarek William Saab in a statement, relates to a PDVSA subsidiary created in Austria in 2006 to bill for oil sales.

The subsidiary used a firm to help set prices, but they were allegedly lowballed to favor some clients.

The activity deprived the state of "at least $4.8 billion" between 2009 and 2015, Saab said.

Ramirez led PDVSA between 2004 and 2014 before becoming Venezuela's ambassador to the UN, a post from which he resigned in early December, saying he did so at President Nicolas Maduro's request.

His present whereabouts are unknown.

But he responded to the accusation on Twitter, calling it "false" and "revealing a tremendous ignorance" because, he asserted, the Austrian subsidiary only verified prices for oil sold and did not sell the oil itself.

One executive of the subsidiary has been arrested, and warrants have been issued against three others.

Saab opened his first criminal probe into Ramirez three weeks ago, accusing him of being part of a scheme to divert $1.5 billion in public funds to Andorra between 2011 and 2012.

Ramirez, 54, has said in interviews that he is a victim of political persecution for his criticism of Maduro's mismanagement of the economy. He denied being involved in corruption.

Venezuela is teetering on the brink of a full-blown default on its massive debt.

Although it sits atop the world's biggest proven reserves of oil, PDVSA's previous role as a government cash cow -- its exports bringing in 96 percent of Venezuela's hard-currency revenues -- has greatly diminished as production has slid and oil infrastructure falls apart.