WASHINGTON — The government is expected to take over Fannie Mae and Freddie Mac as soon as this weekend in a monumental move designed to protect the mortgage market from the failure of the two companies, which together hold or guarantee half of the nation's mortgage debt, a person briefed on the matter said last night.

Some of the details of the intervention, which could cost taxpayers billions, were not yet available, but are expected to include the departure of Fannie Mae CEO Daniel Mudd and Freddie Mac CEO Richard Syron, according to the source, who asked not to be named because the plan was yet to be announced.

Federal Reserve Chairman Ben Bernanke, Treasury Secretary Henry Paulson and James Lockhart, the companies' chief regulator, met yesterday with the top executives from the mortgage companies and informed them of the government's plan to put the troubled companies into a conservatorship.

The news, first reported on The Wall Street Journal's Web site, came after stock markets closed. In after-hours trading Fannie Mae's shares plunged $1.54, or 22 percent, to $5.50. Freddie Mac's shares fell $1.06, or almost 21 percent, to $4.04. Common stock in the companies will be worth little to nothing after the government's actions.

The news also followed a report yesterday by the Mortgage Bankers Association that more than 4 million American homeowners with a mortgage, a record 9 percent, were either behind on their payments or in foreclosure at the end of June.

That confirmed what investors saw in Fannie and Freddie's recent financial results: trouble in the mortgage market has shifted to homeowners who had solid credit but took out exotic loans with little or no proof of their income and assets.

Fannie Mae and Freddie Mac lost a combined $3.1 billion between April and June. Half of their credit losses came from these types of risky loans with ballooning monthly payments.

While both companies said they had enough resources to withstand the losses, many investors believe their financial cushions could wither away as defaults and foreclosures mount.

Many in Washington and on Wall Street hadn't expected Paulson to intervene unless the companies had trouble issuing debt to fund their operations.

This summer, Congress passed a plan to provide unlimited government loans to Fannie and Freddie and to purchase stock in the two companies if needed.

Critics say the open-ended nature of the rescue package could expose taxpayers to billions of dollars of potential losses.

Supporters, however, argue the Bush administration had little choice but to support Fannie and Freddie, which together hold or guarantee $5 trillion in mortgages — almost half the nation's total.

Representatives of Fannie and Freddie declined to comment on the government assistance plan.

Treasury spokeswoman Brookly McLaughlin said officials "have been in regular communications" with Fannie and Freddie, but refused comment saying, "We are not going to comment on rumors."

Concern has been growing that a government rescue of Fannie and Freddie could not only wipe out common stockholders, but also be costly for scores of investment, banking and insurance companies that hold billions of dollars in their preferred shares.

Paulson has been in contact in recent weeks with foreign governments that hold billions of dollars of Fannie and Freddie debt to reassure them that the United States recognizes the importance of the two companies.

The two companies had nearly $36 billion in preferred shares outstanding as of June 30, according to filings with the Securities and Exchange Commission.

Fannie Mae was created by the government in 1938, and was turned into a shareholder-owned company 30 years later. Freddie Mac was established in 1970 to provide competition for Fannie.

A takeover could cost taxpayers up to $25 billion, according to the Congressional Budget Office.

AP writers Martin Crutsinger and Jeannine Aversa contributed to this report.