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 Friday 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 Friday afternoon with the top executives from the mortgage companies and informed them of the government's plan to put the troubled companies into a conservatorship.