The Securities and Exchange Commission has opened a formal investigation into whether a subsidiary of Halliburton Co. was among several companies that paid $180 million in bribes to win a contract to build a natural gas liquefaction plant in Nigeria in the 1990s, the company said yesterday.

The investigation, an escalation of an informal probe begun in February, focuses on the activity of a joint venture that includes Halliburton subsidiary KBR and three other companies. Vice President Cheney was the head of Halliburton from 1995 to 2000, when some of the activities under investigation occurred.

Halliburton officials have recently met with officials from the SEC and the Justice Department and turned over records about the Nigerian project, according to the company's announcement. U.S. officials are examining possible violations of a federal law prohibiting the bribery of foreign government officials.

Halliburton officials said they are also cooperating with a French magistrate who has been examining the case for months.

"While Halliburton does not believe that it has violated the Foreign Corrupt Practices Act, Halliburton's own internal investigation of these matters is ongoing and there can be no assurance that government authorities would not conclude otherwise," the company's statement said.

The Nigerian project, started in the early 1990s, was worth almost $5 billion to TSKJ, a partnership that included a KBR predecessor, as well as companies from France, Japan and the Netherlands.

At issue are payments made to Tristar, a Gibraltar company that had a consulting arrangement with a corporation formed by TSKJ to "administer the contracts and execute the work" in Nigeria, a Halliburton spokeswoman said in response to questions.

KBR, the engineering and construction subsidiary of Halliburton, was formed when Halliburton acquired Dresser Industries Inc. in 1998. It was a combination of Halliburton's Brown & Root and Dresser's M.W. Kellogg Co. Officials from the SEC and Cheney's office declined to comment.