The Dow Jones industrial average followed its worst week in a decade with a 653-point drop on Monday, and US President Donald Trump once again took to Twitter to interject himself into financial markets.
As blue chips sank even deeper into the red after weeks of chaos, Trump tried to assign sole blame for the sell-off to the Federal Reserve, likening the central bank to a golfer who "can't putt".
"The only problem our economy has is the Fed," the president said in a tweet. "They don't have a feel for the Market, they don't understand necessary Trade Wars or Strong Dollars or even Democrat Shutdowns over Borders. The Fed is like a powerful golfer who can't score because he has no touch - he can't putt!"
Monday's decline comes amid a federal government shutdown and repeated attempts by Trump and members of his administration to steady nervous markets, including weekend phone calls to major US banks from Treasury Secretary Steven Mnuchin.
The three major indexes are down for the fourth consecutive session, their worst streak in more than three years. The tech-heavy Nasdaq Composite that was key to the current bull market resides in bear territory. A bear market is 20 per cent off recent highs.
The Nasdaq declined 140 points, about 2.2 per cent on the day.
The S&P 500 joined the Nasdaq in bear territory, down 65 points, or 2.71 per cent, to close at 2,351. All 11 sectors of the S&P were down and are negative for 2018.
Utilities and energy shares were the biggest drags Monday, with energy stocks under pressure from a 35 per cent decline in oil prices since October.
December is usually a healthy month for stocks, but this month has been the worst since 1931.
"There's carry-over selling from weakness last week combined with lighter-than-average volume, that allows for opportunistic traders to push share prices lower without much opposition," said Sam Stovall, of CFRA Research.
Mnuchin shook financial analysts, bankers and economists on Sunday by issuing an unusual statement declaring that the nation's six largest banks had ample credit to extend to American businesses and households.
Mnuchin made the statement on Twitter after calling the leaders of the six banks, seeking to address an issue that had attracted little concern ahead of the treasury secretary's tweet.
"He raised the spectre of concern in one of the most solid areas of the US financial system, the solvency of the banks," said Washington, DC, investor Michael Farr. "The banks are as solvent as they have ever been. He went to the heart of the economy's strength and created doubt."
Mnuchin followed that up Monday by scheduling a conference call of the "Plunge Protection Team," which was a committee created following the 1987 stock market crash to help stem steep drops in the market.
The members of the committee include the heads of the Federal Reserve, the Securities and Exchange Commission and the Commodity Futures Trading Commission.
Ed Yardeni, president of Yardeni Research, said the current pullback was a reaction to the Federal Reserve moving too far, too fast to raise interest rates.
"It's not only the stock market," Yardeni said. "We are seeing stress in the credit markets and the commodity markets, signalling that the Fed might have been naive in believing after several years of abnormally easy monetary policy, that they could reverse or normalise policy in just a few years. The markets want the Fed to normalise at a much slower pace."
Trump has long claimed personal credit for market gains and sees a strong economy as central to his re-election hopes in 2020. But the president has obsessively monitored recent gyrations and aides say he has grown furious about the declines of the past week and fearful of a recession.
Trump has singled out his handpicked chairman of the Fed, Jerome Powell, for blame, insisting that the bank's interest rate hikes are responsible for the downturn.
In a November 27 interview with the Washington Post, Trump said: "I'm not happy with the Fed. So far, I'm not even a little bit happy with my selection of Jay. Not even a little bit."
Trump suggested he had a better feel for the markets than Powell, even though the Fed chairman has spent decades working at investment banks and the Treasury Department, and Trump, as a real estate developer, ran a number of companies which went bankrupt.
"I have a gut, and my gut tells me more sometimes than anybody else's brain can ever tell me," Trump said in the interview.
The economy and financial markets - stocks, bonds, commodities - are made up of a constellation of factors that are not synchronised. The current stock market pullback may have nothing to do with US banks. The US economy's fundamentals are sound, even though the stock market is verging on bearish territory.
"The president believes he can tweet himself out of this bear market, and he is just making it worse," said Jared Bernstein, former economic advisor to Vice President Joe Biden. "History is littered with economies that have been brought down by strong-man, populists muscling their central banks.
In Mnuchin's statement, which was posted while he was vacationing in Mexico for the holidays, the Treasury chief said the executives told him "they have ample liquidity available for lending to consumer, business markets, and all other market operations." Mnuchin said the executives added "that they have not experienced any clearance or margin issues and that the markets continue to function properly."
Mnuchin's Sunday phone calls followed weeks of erratic markets, often bouncing on Trump administration actions on US-China trade, criticism of Powell and putting pressure on Saudi Arabia to pump more oil to keep prices low.
"It used to be during time of crises, investors asked, 'What will the government do for us.' Now they are questioning, 'What else will they do to us,'" Stovall said.