Reflecting concerns about the economy, crude oil prices fell 2.7 percent, to $86.59 a barrel. OPEC, the oil-producing cartel, on Thursday called an emergency meeting for Nov. 18 to consider cutting an output in an attempt to arrest the 41 percent drop in oil prices in the last three months.

Asian markets also reacted strongly to the turmoil. In Japan, the Nikkei 225 index was down 10 percent on Friday morning, after having plunged 9 percent on Wednesday. The Hang Seng index in Hong Kong fell 7 percent and the market in Singapore was down 6.8 percent.

What has prompted the late-day sell-offs recently is a subject of intense debate and conjecture, even among market professionals, who also have been unnerved by the free fall of the week.

Some attribute it to mutual funds’ waiting until midafternoon to execute sell orders from a growing number of investors who are cutting their exposure or bailing out of the market altogether. Others say that hedge funds, which have leveraged returns in recent years by using borrowed money, are having to sell holdings to raise collateral against their borrowings.

Still others say computerized trading, which has grown significantly in recent years, often kicks in later in the day, when certain thresholds are breached.

But whatever the reasons for the late-day plunges, what is driving the market down is a lack of confidence by investors, who are skeptical that the many measures taken by the government to rescue the financial system will work. Moreover, they worry that the government’s trotting out a new initiative every day or two is a sign that maybe the situation is worse than many thought.

Some who have held on until now are starting to sell. Since the start of the year, investors have removed more than $81 billion from stock mutual funds, with nearly 40 percent of that coming in the last six weeks, according to AMG Data Services.