Mark Barton's application for a desk and a terminal at Momentum Securities shimmered with the cocky optimism required of any day trader. Barton claimed a net worth of $750,000 and plunked down $87,500 to start trading stock.

On June 9, Barton activated his account. On Tuesday, he got the news that eventually comes to more than three-quarters of day traders, the adrenaline-addicted, thrill-seeking cowboys of the electronic range. Margin call. Cough up the cash you owe the brokerage, or your account will be closed. Momentum's Atlanta office manager interrupted Barton's trading -- the ex-chemist was frantically trying to dig out of heavy losses -- to inform the investor that to keep his account open, he had to pay. Now.

That day, police say, Barton killed his wife and stuffed her in a closet.

Barton immediately wrote a check for $50,000. On Wednesday, Momentum's manager said the bank would not honor the check.

That day, police say, Barton killed his two children.

On Thursday afternoon, Barton was back at the office, promising to transfer $200,000 to Momentum from an account he had at Charles Schwab. The office manager was at lunch, but Barton reached him on his cell phone. "I'm good for the money," Barton said, according to someone who spoke to the manager later.

The manager said he would review Barton's paperwork when he got back. Wait there, he said.

Barton stepped into Suite 310, the trading room, a small office with about 20 desks and computers. He wandered about, calmly greeting his fellow traders.

"Hey, how are you doing?" one trader asked Barton.

"I'm fine," Barton said.

Then he pulled out his guns and began shooting. Blood splattered onto computers. Traders dove to the floor. Four men died.

Susan Strobel was checking e-mail at her desk in the insurance office next door when she heard the shots. She retreated into a back office, where she found a young man on the floor, face down, moaning in pain. Trader Bradley Schoemehl, 24, had been shot in the right shoulder and above the waist, Strobel said.

"Mark -- he's lost all his money," Schoemehl told Strobel. (Local hospitals, at the request of victims' relatives, are not divulging information about individuals Barton shot, but said their conditions ranged from satisfactory to good.)

In his eight weeks at Momentum, Barton had suffered $105,000 in losses, according to sources with knowledge of his account.

Barton's troubles beyond the financial realm have already revealed a more complex genesis of his murderous rampage, but the shootings in Atlanta nonetheless have shaken the small but burgeoning world of investors who spend long, intense days glued to computer terminals, their fingers at the clicker, their eyes constantly scanning for the fluctuations in stock prices that -- over hundreds of trades -- could make them rich.

At trading rooms from Atlanta to Arlington, the several thousand amateur investors who dream of riding the technology boom to near-instant wealth without ever having to don a tie or create a product have suffered a jittery few weeks. The stocks that have reliably soared for them have been drooping since April.

Yesterday, as traders jockeyed to squeeze their wagers between their own emotional strain and the demands of a suddenly curious public, the impact of Barton's bullets was evident:

At Net Trade in Arlington, a blackboard listing a dozen tips for successful day trading ("Do not overtrade," "Cut your losses decisively") showed a late addition: "Do not blame others for losses."

The high-stakes, high-speed stress of day trading -- at Net Trade, for example, each trader makes an average of 500 trades a day -- is a drug that had attracted but a small club of investors only a decade ago. It wasn't until the advent of real-time instant online trading and the boom in Internet stocks last year that the phenomenon really took off.

The field has exploded with such ferocity -- most estimates put the number of traders somewhere between 3,000 and 7,000, working in about 100 centers nationwide, with many more trading from home -- that federal and state regulators have sought unsuccessfully to get a handle on the dramatic risks and wild volatility endemic to the practice.

At least two states have sued day-trading firms for false advertising, fraud and failing to screen customers. When Barton signed up at Momentum, he was required to read and sign a letter from Securities and Exchange Chairman Arthur Levitt Jr. declaring day trading a risky venture.

Day trading was particularly dangerous for someone like Barton because he did not man his account daily, according to sources familiar with his trading. Day-trading experts preach that traders must buy and sell stocks full time if they're going to do it at all. They also encourage investors to close out their accounts by the end of each day so they are not vulnerable to overnight swings in stock prices.

But Barton showed up at Momentum only sporadically and often left with his accounts open. While he was out, his account plunged and peaked. By Tuesday, he was down $105,000 trading on volatile Internet stocks such as bookseller Amazon.com. And it emerged that Barton had also racked up enough debt across the street at All-Tech for that company to shut down his account last April.

"He had become very compulsive about trading," said someone who knew Barton.

Momentum and other day-trading companies say they are getting tougher about extending credit and deciding which new customers to accept. Momentum requires an initial deposit of $50,000.

And All-Tech's Web site features this notice on its home page: "Warning: Active electronic stock trading is risky."

At Traders Advantage, a two-year-old Atlanta day-trading firm, president Ira Lazar said that in addition to requiring a minimum investment of $25,000, he vets potential clients to make sure they can weather the risk. "If a person comes in and he has a wife and two kids and makes $25,000, we're not going to let him open an account at all," he said.

Barton's Momentum application listed his annual income at $85,000.

Lazar defended his trade against the argument that it is taking advantage of uninformed investors' money lust. "It's not gambling," he said. "It's informed speculating."

Even so, over time, most day traders will end up with less than they started with, according to some of the trading firms themselves.

Like most day-trading firms, Momentum was an aggressively casual workplace, where traders arrived unshaven and in shorts. Office administrator Kevin Dial -- who was killed by Barton -- would supply pizza and ice cream to enable customers to stay through the lunch hour and keep trading, said Aetna insurance agent Dan Kirk, who works next door.

"That's how you make money," Kirk said. "They would keep 'em in there, keep 'em trading." Most day-trading firms charge their clients a rental fee for the computers and desks and a per-trade fee of $25 or less, depending on the number of trades they're making.

Barton traded at Momentum on 15 days over the past two months; on four of those days, he ended up in the black, according to a letter from Momentum president James Lee to SEC chairman Levitt.

Losses are part of the game for day traders, most of whom have seen one of their colleagues get at least momentarily frantic. At Net Trade's nerve center, a windowless chamber decorated with framed Russian rubies, the young men who spend their days together recall a moment four months ago when they had an unnerving taste of what can happen when losses mount.

A struggling day trader became obsessed with the idea that people were watching him and even accused co-workers of being out to get him. It was an extreme case of the paranoia that many day traders say they can feel when they are deep in the zone, submerged in the real-time river of information tracing every minute movement of stocks. Traders described a fear that someone -- another stock broker, the entire stock exchange, your office manager -- is watching your every move.

"I said two months ago that something like this was going to happen," said Lawrence Black, a baby-faced trader who has set up his battle station with six monitors, a cell phone, a cordless phone and a fortune-telling eight-ball. "I think it's inevitable. People aren't getting the proper training to do this."

Black, 27, believes the business is expanding too rapidly, with too many inexperienced day traders jumping in without the ability to tolerate the stress. Of Barton, he said, "Obviously the guy had issues. It's stressful, but it doesn't make you kill people."

"The successful people who do this don't always want to win," said Stephen Boren, a psychologist who dabbles in day trading at Net Trade and has been studying what makes a successful day trader. "They want to master a skill. It's a stressful profession. It's a lot like being in law enforcement or a firefighter."

Losing lots of money can put anyone over the edge, Boren said. "But not everyone would go on a shooting spree."

In the online chat rooms where traders commiserate and gloat between trades, investors yesterday trashed the news media both for hyping day trading and for creating too casual a link between the practice and Barton's crimes.

"Too many common inexperienced folk out there are playing this game and giving up their careers to turn a quick buck," one woman wrote on the Silicon Investor Web site. "This has a negative impact on the stock market and can put personal lives in ruin. The media should downplay the glamorization of day trading."

An Atlanta investor, spooked that he had made his first trade just down the street from the shooting site, worried that an economic downturn could prompt similar horrors. "I hope we don't hit a bear market, else we'll see more of this," he wrote.

Barton's shooting frenzy created a horrific window to the volatility of day trading. And Friday afternoon, even as workers removed bloodied carpeting and investigators sought to reconstruct Barton's final days, Momentum Securities received a coda to the week's wild swings: Barton's bank informed the brokerage that the $50,000 check he had written on Tuesday had, contrary to first indications, cleared.

Dugan reported from Pennsylvania, Nakashima from Atlanta and Fisher from Washington. Staff writers Emily Wax and Jerry Knight in Washington contributed to this report.