Wall Street’s angst over the ongoing fallout from the credit crisis made for a turbulent end to a volatile week today — stocks tumbled, soared, and then turned south again as investors tried to assess the dangers faced by the country’s biggest mortgage financiers, Fannie Mae and Freddie Mac.

The Dow Jones industrial average, which traded down more than 250 points in the session, briefly moved into positive territory today before resuming its decline. The blue chips also fell below 11,000 for the first time in two years before recovering.

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A new high for oil prices above $147 a barrel also weighed on stocks.

Investors’ focus was on the fate of the government-chartered companies. Shares of Fannie Mae and Freddie Mac fell sharply during the week on concerns about their stability. Wall Street is worried that a collapse of the two financiers would cause further shock to the financial system, and trigger more losses to banks and brokerages with significant holdings of mortgage-backed securities.

The well-being of Fannie Mae and Freddie Mac is crucial because they hold or guarantee about $5 trillion worth of mortgages. Their troubles are just the latest depressing turn in a year-old credit crisis that shows no sign of ending, disappointing some stock traders who thought just months ago that the worst was perhaps over.

According to preliminary calculations, the Dow fell 128.48, or 1.14%, to 11,100.54 after having fallen to 10,977.68. It last traded below 11,000 on July 25, 2006.

Broader stock indicators also logged declines. The Standard & Poor’s 500 index fell 13.90, or 1.11%, to 1,239.49, and the Nasdaq composite index fell 18.77, or 0.83%, to 2,239.08.

Today’s drop meant Wall Street moved squarely into a bear market, which is defined as a 20% drop from a recent peak. The Dow is down 21.6% from the record closing high of 14,164.53 it reached in October. The S&P 500 is down 20.8% and the Nasdaq is off 21.7%.

Oil, meanwhile, extended its move into record territory, rising as high as $147.27 amid tensions between the West and Iran. Light, sweet crude for August delivery settled up $3.43 at $145.08, slightly below a record close of $145.29 a barrel set more than a week earlier.

Bond prices fell sharply as investors worried a bailout of Fannie Mae and Freddie Mac could dent the government’s credit rating. Ordinarily, bonds are seen as a safe haven during stock market pullbacks. The yield on the benchmark 10-year Treasury note, which moves opposite its price, rose to 3.95% from 3.80% late yesterday. The dollar was mixed against other major currencies, while gold prices rose.