Just as CIT Group Inc. (CIT) finished beating back a revolt of its junior bondholders, another creditor picked a fight Monday with the embattled century-old commercial lender - corporate raider Carl Icahn.

In a blistering letter to CIT's board, the billionaire investor complained that a proposed solution hammered out by the cash-strapped company and its largest creditors is too costly and detrimental to CIT's smaller bondholders.

In place of that deal, he offered to underwrite a $6 billion loan to the firm that he said would carry half the upfront expense of similar new funding CIT has arranged for itself. If CIT does not want to do business with him, Icahn said other banks would do the same.

Icahn has a reputation for opportunistically - and publicly - zeroing in on companies in distress. In May 2008, for instance, he launched a proxy fight to unseat Yahoo Inc.'s (YHOO) board after that wobbly web portal rejected a takeover by Microsoft (MSFT).

Now that Icahn has thrown himself into CIT's restructuring, there's no telling what's next for the cash-strapped lender to small and midsized businesses.

"This throws a significant monkey wrench into what the board is trying to do with the two-tier vote" on CIT's restructuring, said Chris Munck, who trades CIT bonds at B. Riley & Co.

Icahn, who refers to himself as CIT's largest creditor, declined to comment on the size of his bond holdings or at what discount he bought them; someone familiar with the matter said Icahn has accumulated various classes of short- and long-term notes over the past few months.

In a written statement Monday afternoon, CIT said it planned to ask Icahn for more information about his proposal, saying that his letter, which he immediately made public, marked the first time it had heard of his interest.

The timing of Icahn's assault is curious, said Brian Charles, an analyst at R.W. Pressprich in New York. Indeed, it comes after CIT, which is teetering on the brink of bankruptcy, late Friday amended the terms of a sweeping debt exchange, asking investors of around $31 billion in bonds to cut the debt by at least $5.7 billion and extend the debt maturities.

Bondholders are also voting on a prepackaged bankruptcy plan, which many see as the more likely outcome.

"I'm not sure where Icahn is going with this. [But] he does have a point - even if it [may be] self-serving - the company should allow all bondholders to participate in any expansion of the senior secured facility, regardless of how they vote the restructuring plan," Charles said.

"At this point it seems like game theory squared and squared again," Charles added.

Under the revised terms of the restructuring plan, New York-based CIT will increase the amount of equity offered to holders of its subordinated debt and include bonds that mature after 2018 that previously weren't part of the exchange offer or reorganization plan that was announced Oct. 1. The company is also proposing that maturities on new notes issued as part of the exchange be shortened by six months. It will also increase the interest on new debt offered to holders of bonds sold by a Canadian unit of CIT, the firm had said on Friday.

The company is also looking for new leadership, as current CEO Jeffrey Peek has said he will step down at the end of this year. While the changes outlined Friday make the debt exchange plan more palatable to some bondholders, they do little to address CIT's long-term outlook.

Shares closed up 8% at $1.21. The stock is down 73% this year.

-Kate Haywood, Dow Jones Newswires; 212-416-2218; kate.haywood@dowjones.com

(Maxwell Murphy and Kevin Kingsbury contributed to this article.)