Leucadia National Corp., a New York buyout firm, has expressed interest in acquiring a controlling stake in MCI Inc., raising new questions about the telecommunications giant's future just three months after it emerged from bankruptcy.

MCI issued a statement yesterday saying the takeover specialist plans to ask the Federal Trade Commission and the Department of Justice for antitrust approval to acquire at least 50 percent of MCI's common stock. Based on yesterday's closing price, a stake that large would cost Leucadia at least $2.75 billion.

Leucadia declined to comment, and it was not clear yesterday that the investment firm had lined up financing. It took Leucadia two years to buy a much smaller telecommunications firm, according to regulatory filings. It also could not be determined if the firm had even formally begun the process of seeking the necessary government approvals.

The takeover bid comes as Ashburn-based MCI continues to struggle financially after eliminating about $35 billion in debt through the bankruptcy process. Last month, MCI announced it would lay off another 2,000 workers, bringing the total job reductions in the last year to more than 15,000.

MCI filed for bankruptcy protection in July of 2002 after a massive accounting scandal. The telecommunications giant has 15 million customers and is now the nation's third-largest long-distance company behind Verizon Communications Inc. which has approximately 16 million customers. AT&T Corp. had 30 million customers as of the end of 2003.

Leucadia specializes in acquiring troubled businesses and turning them around. Its portfolio includes real estate, mining, petroleum and telecommunication interests. Among its holdings are two relatively small telecommunications companies, WilTel Communications Inc. and ATX Communications Inc. Leucadia acquired its controlling stake in both companies while they were in bankruptcy.

MCI declined to comment on Leucadia's intentions beyond a press release stating that the investment firm was applying for regulatory approval. Officials at both the FTC and the Justice Department said such applications are confidential and referred calls to the companies.

Investors responded by bidding up the stocks of both companies. Shares of MCI are not currently traded on a major exchange, although the stock does change hands through the so-called pink sheets, an electronic market that does not require trading companies to meet any financial criteria. Shares of MCI (MCIA) shot up $2.45, or more than 16 percent, to close yesterday at $17.05. MCI said yesterday that it plans to begin trading its shares on the Nasdaq electronic market on Wednesday, according to company spokesman Peter Lucht.

It is not unusual for shares of an acquiring company to fall on the day that a major deal is announced, as investors worry about taking on new debt. Instead, Leucadia's shares rose 2 percent, or $1.12, to close at $50.05. "The interesting thing is that both Leucadia and MCI have popped up; obviously people think this would be a good thing," said James Andrew, vice president of Adventis, a telecommunications consulting firm.

Some industry analysts regarded MCI's announcement as a sign that the company is about to be put on the auction block.

"We think that basically this sets the wheels in motion for MCI to be sold," said Romeo A. Reyes, a telecommunications analyst with Jefferies & Co., which owns MCI's stock. Verizon and Sprint Corp. could be potential bidders, Reyes said.

But other analysts played down the idea that a wide-scale bidding war is imminent. Drake Johnstone, a telecommunications analyst with Davenport & Co., noted that the two of the industry's largest players -- SBC Communications Inc. and BellSouth Corp. -- are already distracted by the proposed merger of Cingular Wireless and AT&T Wireless. Cingular is a joint venture of SBC and BellSouth.

Verizon, meanwhile, would likely face tough regulatory scrutiny if it attempted to acquire MCI because the combination would bring together two of the top three long-distance providers. "There would certainly be antitrust issues if Verizon were to seek to acquire MCI," said Scott Harris, a telecommunications attorney in Washington.

Leucadia could be considering a plan to break up MCI, selling its parts to the highest bidder. Among the most likely holdings to be sold are MCI's ailing residential long-distance business and its European operations, Andrew said. That would leave MCI with its most valuable assets: an impressive roster of large business customers and one of the nation's largest Internet networks, according to Andrew.

Leucadia acquired its largest telecommunications asset, WilTel, over a two-year period, spending approximately $780 million to buy all outstanding shares during 2002 and 2003. WilTel's major assets are 30,000 miles of fiber-optic cables that stretch across the country. In addition, it has local fiber-optic networks in 36 cities including Washington, New York and Los Angeles. WilTel also leases undersea links to several European countries. The WilTel assets could be combined with MCI's vast domestic networks to offer local, long distance and international data and voice connections, analysts said.

Long-distance giant SBC Communications Inc. is WilTel's largest customer by far, accounting for 65 percent of total sales, according to a Leucadia regulatory filing.

Leucadia's officers rarely speak to the media. The two largest shareholders are Paul M. Dougan, who has been president and chief executive of Equity Oil Co., a Salt Lake City oil and gas producer that is in the process of being acquired by Denver-based Whiting Petroleum Corp. He controls 13.2 percent of Leucadia's stock and has been a board member since 1985.

Leucadia President Joseph S. Steinberg, a firm co-founder, owns 12.9 percent of the company and has been a director since 1978. He also is a director of a related company that owns and manages manufacturing concerns, and White Mountains Insurance Group Ltd., a publicly traded insurance holding company.

The company keeps a low profile and only recently published a Web page that provides little more than links to its Securities and Exchange Commission filings.

In addition to its mining, petroleum, health insurance and real estate holdings, Leucadia has entered partnerships with other investors including Warren Buffett's Berkshire Hathaway. The two firms formed a joint venture known as Berkadia that provided financing to Finova Group Inc., a financial services group.