Stocks were sharply lower on Thursday, but staged a sudden comeback in the final 90 minutes of trading.

The major indexes have been deep in the red for much of trading session.

Near 2:38 p.m. ET, the Dow was down as many as 408 points.

Crude oil was also getting slammed, with West Texas Intermediate crude futures in New York touching a 12-year low of $26.14 a barrel.

But then oil surged along with stocks after a Wall Street Journal reporter tweeted that, according to the UAE energy minister, OPEC is ready to cooperate on a production cut. 

The Dow regained about 80 points within 15 minutes.

Investing.com

Earlier, the Dow was dragged down by Boeing, which was off as much as 10% after reports the SEC is looking into its accounting practices for its recently-launched "Dreamliner" jet. 

The Nasdaq, which contains many of the largest public tech companies, was approaching a bear market — defined as a 20% drop from recent highs. 

And, the S&P financial index fell 3%, as major banking stocks including Citigroup and Bank of America were selling off. Both stocks were as much as 6% lower. 

Meanwhile, investors were piling into safe-haven assets in "risk-off" trades. Gold futures were up more than 4%, or a whopping $55 an ounce to as high as $1,263.40, the strongest in a year.

US Treasuries — another safe-haven asset — were also getting a huge bid, pushing the yield on the benchmark 10-year note down to as low as 1.53%, a level it most recently touched in late 2012.

Yellen's testimony

Federal Reserve Chair Janet Yellen concluded her semiannual testimony on Capitol Hill this afternoon.

She said negative interest rates are not off the table, and the Fed is studying whether they would be an effective policy tool if they are needed.

On Wednesday at her first appearance, Yellen noted that the Fed was closely watching developments in financial markets and foreign economies for possible downside risks to the economy. She did not rule out or confirm an interest-rate hike in March, but she said the Fed was willing to slow down its path of rate increases if the economic data indicated doing so would be appropriate.

In all, her assessment of the economy was more or less unchanged: The economy is still expanding at a moderate pace, and the labor market is robust. Still, the Fed has been watching all that's happening in financial markets and may adjust its policy plans if the economic data doesn't hold up.

Meanwhile, the Swedish National Bank cut its key interest rate further into negative territory overnight to boost domestic demand.

And once again, banking stocks in Europe took a hit, with Societe Generale shares dropping 12% after the bank reported earnings that were below expectations. Deutsche Bank shares tumbled earlier this week as investors became concerned about its ability to meet its credit obligations.

Overall, European stocks fell Thursday to their lowest level since September 2013.

In economic data, first-time claims for unemployment insurance (initial jobless claims) dropped 16,000 to 269,000 last week, the lowest level of the year. 

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