As bad as the day was, even worse was the cumulative effect of a historic run of declines: The Dow suffered a triple-digit loss for the sixth day in a row, a first, and the average dropped for the seventh day in a row, a losing streak not seen since 2002.
"Right now the market is just panicked," said David Wyss, chief economist at Standard & Poor's in New York. "Nobody wants to take on any risk. Everybody just wants to get their money and put it under the mattress."
It all took place one year to the day after the Dow closed at its record high of 14,164. Since that day, frozen credit, record foreclosures, cascading job losses and outright fear have seized the market and sapped 39 percent of its value.
Paper losses for the year add up to an staggering $8.3 trillion, according to preliminary figures measured by the Dow Jones Wilshire 5000 Composite Index, which tracks 5,000 U.S.-based companies representing almost all stocks traded in America.
It was the second straight day that Wall Street was rocked by a final-hour sell-off, but this one was particularly shocking.
Most of the day was relatively calm, and the trading floor was quieter than usual because of the Jewish holiday of Yom Kippur. Wall Street awoke to news the federal government was brandishing a new weapon against the financial crisis - considering seeking an equity stake in major U.S. banks in order to stabilize them.
But that step appeared to be as ineffectual as the others Washington has rolled out in recent weeks, including a $700 billion bailout of the financial industry, a coordinated interest rate cut by central banks around the world and direct lending by the Federal Reserve to private companies to provide them with short-term cash.
Acquiring a stake in the banks would be yet another startling intervention by the government in the free market, but economists said President Bush was left with little choice because of the credit markets, where tight lending has choked off the everyday cash that is the lifeblood of the economy.
"In normal times, this would be out of the question, but in the present dire situation, I think the government should be employing all the powers that it can," said Sung Won Sohn, an economics professor at California State University, Channel Islands.
Wall Street has been teetering on the brink of panic for a month now, vulnerable to any bad news. Thursday's sell-off was triggered when a major credit rating agency put General Motors Corp. and its finance affiliate under review to determine whether it should be downgraded.
Stock in GM, one of the 30 components of the Dow Jones industrials, lost 31 percent of its value and closed at $4.76 - its lowest in more than half a century, since the Korean War began.
For the Dow, it has been nothing short of a free fall:
-The average is down 2,338 points, or 21 percent, in the last four weeks, since the Lehman Brothers bankruptcy escalated a long-running credit crunch into a full-fledged crisis.
-The point decline Thursday was the third-worst in Dow history. The worst, 778 points, came less than two weeks ago.
-Of the last 19 trading days, there have been 11 triple-digit losses - including the unprecedented six straight. The six gains have all been triple-digits, and only one of them was enough to make up the losses of the day before.
-The Dow now stands only about 1,300 points above its lowest close of the bear market that followed 9/11. In a market as volatile as this, that gap can be closed in a couple of trading days, or less.
In fact, triple-digit declines can happen almost in an instant.
On Thursday, the Dow was above 9,200 after 1:30 p.m. and still above 9,000 after 3 p.m. The pressure to sell was so intense that the Dow kept dropping precipitously for 10 minutes after the 4 p.m. closing bell as the day's losses were tabulated.
In percentage terms, the drop in the Dow exceeded the day the markets reopened after the Sept. 11, 2001, terrorist attacks. It was not close to the 22.6-percent decline on Black Monday in 1987, the last stock market crash.
Still, it is becoming increasingly clear that Washington has ever fewer places to reach in its toolbox to stop, or perhaps even slow, the crisis. Among the options still left are buying up foreclosed properties and making direct loans to homeowners, both of them hard for free-market supporters to swallow.
Speaking in the afternoon before the market closed, President Bush told an audience on the South Lawn of the White House that the economy was going through a "very touch stretch." But, he said: "I'm confident in our economy's long-term prospects."
After the market closed, the White House said Americans should remain confident despite the market plunge, and President Bush planned to speak from the Rose Garden on Friday morning - though he was not expected to unveil any new policy proposals.
"The Treasury Department is moving quickly to use new tools to improve liquidity, which is the root cause of this problem," White House press secretary Dana Perino said. "Americans should be confident that every effort is being taken to stabilize our markets."
The broader stock indicators registered similar declines to the Dow's. The Standard & Poor's 500 index fell 7.6 percent to the 909 level, and the Nasdaq composite index fell 5.5 percent to 1,645.
Meanwhile, the credit markets remained stubbornly locked-up. The benchmark rate that banks charge each other for loans, known as Libor, rose to 4.75 percent from 4.52 percent a day earlier, signaling banks are still afraid to make loans because they worry they won't be paid back.
"The story is getting to be like that movie Groundhog Day," said Arthur Hogan, chief market analyst at Jefferies & Co. "Everything we're seeing is historic. The problem is historic, the solutions are historic, and unfortunately, the sell-off is historic. It's not the kind of history you want to be making."
Adding to Wall Street's nervousness, a ban on short selling - a process in which investors borrow shares of stock and essentially bet the value will fall - expired.
With three and a half weeks before voters elect Bush's successor, there was also no immediate comment on the Wall Street action from the presidential candidates, Democratic Sen. Barack Obama and Republican Sen. John McCain.
Earlier in the day in Dayton, Ohio, Obama took aim at McCain's plan to have the government absorb the full cost of renegotiating mortgages for borrowers under strain from the dramatic decline of the values of their homes.
McCain rolled out the idea at the second presidential debate earlier this week, a forum in which he also told voters it was important to have a steady hand in the White House during a time of economic crisis.
AP Economics Writer Martin Crutsinger reported from Washington. Associated Press writers Tom Raum in Washington and Patrick Rizzo in New York contributed to this story.