Updated from 3:48 p.m. ESTStocks closed sharply lower again Thursday, as a report that the al Qaeda group was behind a group of explosions in Madrid earlier in the day multiplied losses, pushing the Dow Jones Industrial Average 5.6% below its 2004 high set in February. The Dow tumbled 168.51 points, or 1.64%, to 10,128.38, its biggest one-day loss since last May. The S&P 500 slipped 17.10 points, or 1.52%, to 1106.79, also its biggest loss in 10 months and a new low for the year, while the Nasdaq fell 20.26 points, or 1.03%, to 1943.89. Over four losing sessions this week, the Dow is down 468 points, or 4.4%, while the Nasdaq has given up 104 ponts, or 5%. Volume on the New York Stock Exchange today approached 1.9 billion shares, while 2.2 billion shares changed hands on the Nasdaq. Decliners outnumbered advancers by about 3 to 1 on both markets. The late swoon Thursday came after reports that a London-based Arabic language newspaper said al Qaeda had claimed responsibility for a group of explosions that rocked three train stations in Madrid, killing at least 190 people. "The reports about al Qaeda being involved in this attack in Spain really creates a lot of fear out there," said John Hughes, an equity strategist at Shields & Co. "You're oversold on the Nasdaq and you're almost oversold on the Dow and the S&P, but that rumor late in the day like this has some people panicked. Is there another attack coming tonight somewhere, or something like that? Who knows? "But balance that against the fact that we are oversold," Hughes added. "If we continue down like this, we should be able to get some kind of lift in here in the next couple of days." The latest losses come amid a growing consensus that the major indices are undergoing a long-awaited correction following sharp gains in 2003.