R2 Investments LDC vows to again sue Carl Icahn if XO Holdings files for bankruptcy for not being able to repay its $461 million debt.

R2, which holds a 6.6 percent stake in XO, accused Icahn – for the second time this year – of deliberately waiting to refinance the carrier’s credit load so he can snap up the rest of its money-making resources. The group said Icahn should have gone after new financing terms in 2005 when the economy was soaring.

“Now,” R2 wrote on June 12, “your XO debt matures in 2009 and the credit markets are in turmoil. We are very concerned given your past actions that as your XO debt matures, you will continue to use your position as chairman of the board and majority shareholder of XO to try to obtain all of XO’s revenue-generating assets for yourself to the detriment of XO’s minority shareholders.”

R2 is affiliated with investment banker Geoffrey Raynor, who is based in Fort Worth, Texas and tends to focus on distressed technology companies. Telecom industry watchers previously have wondered why Icahn might want to operate XO at a loss. They’ve speculated that Icahn – who owns 60 percent of XO’s common stock and 90 percent of its debt – gets a cushy tax break when XO’s losses have offsetting gains.

A so-called activist investor, Icahn this year has trained his attention on Motorola Inc.’s troubles and tried to push a Microsoft-Yahoo! pairing, a deal that was scrapped for good as of yesterday.

R2 in its June 12 letter made several demands: it wants people affiliated with Icahn to resign from XO’s board; it wants them to put their common stockholdings in a blind voting trust; it wants to ensure Icahn affiliates don’t make any personnel or compensation decisions; and it wants them to stay out of shareholder, board and management meetings.

R2 sent a similar letter to XO’s board in February.

XO’s stock was holding steady at 75 cents on Friday. The company trades on the Over The Counter Bulletin Board and its shares have dipped to 47 cents over the past 52 weeks. Company executives said they don’t comment on shareholder disputes.