Authorities believe Mark Barton killed his wife and two children in their Stockbridge, Ga., apartment. (AP)

Mark Barton, a 44-year-old former chemist who a fellow stock trader said was upset about investment losses and marital problems, allegedly walked into the branch offices of All-Tech Investment Group in the affluent Buckhead section of Atlanta just before 3 p.m., spoke briefly with the office manager, then pulled out 9mm and .45-caliber handguns and began shooting.

Barton, who had not traded at the firm since April, first shot the firm's two office managers and their secretary at close range before firing indiscriminately into the room where 30 to 40 people usually use terminals to trade stocks, sending people diving under desks and running for the door, said officials at All-Tech headquarters in New Jersey. A computer was thrown through a window by those trying to escape.

John Cabrer, who works in an adjacent building in the complex, said he heard witnesses tell police that the gunman said, "I hope this doesn't ruin your trading day."

Nell Jones, an All-Tech employee, described Barton as "calm and determined." Jones was not wounded, but was treated for shock at an Atlanta hospital.

Leaving five dead, the 6-foot-4 Barton then left the building and crossed the street to a neighboring building in the Piedmont Center office complex, where he killed four more people at another day-trading firm, Momentum Securities, Atlanta officials said.

"There's an awful lot of blood everywhere," Atlanta Mayor Bill Campbell said at a news conference.

Cabrer, 30, who works at the State Tollway Authority, was among the first to arrive after Barton fled. "I looked down to my left and one of them was in a fairly large pool of blood. The other one was in a corner. They were both hunched over in a fetal position," he said last night. He tried to resuscitate one man unsuccessfully and found two others dead before paramedics arrived. One wounded man asked Cabrer to call his wife, but Cabrer was only able to reach an answering machine.

The shooting ranked as one of the country's most deadly office massacres, and the latest in a series of shootings that have shocked Americans.

"These are unspeakable tragedies," said Campbell, who put himself in the center of today's events by holding two televised news conferences to report breaking developments. His city is one of several that have sued gun manufacturers over fatal shootings. "From Columbine to Atlanta, these things occur," Campbell said. "We do not know why."

Police and SWAT teams searched the Buckhead area for Barton for hours after the shooting, going through four buildings in the office complex floor by floor, escorting to safety workers who were trapped and in many cases hiding under their desks while the hunt for Barton continued.

Shortly after 8 p.m., Barton's green Ford minivan was pulled over by police at a gas station in Cobb County, Ga., north of Atlanta. He then shot himself, Campbell said. "There is no information that he attempted to flee. He was pulled over and then apparently killed himself at that time," Campbell said.

The bodies of Barton's wife and two children were found in their home in Stockbridge, Ga., an Atlanta suburb. They had apparently been killed earlier in the week. The children were initially discovered today in their bed by a welfare worker who responded to a call from someone concerned about the family, Henry County Police Chief Jimmy Mercer said at a news conference. Police later found Barton's wife's body in a closet. Mercer said the cause of death was blunt-force trauma.

Barton had also been a suspect in the 1993 murders of his then-wife and mother-in-law, who were hacked to death in a campground in northeastern Alabama.

"We ask all the people of Atlanta to pray for the families of all those who have been killed or wounded," said Campbell, who personally took charge of briefing reporters throughout the afternoon and evening. "This is something very difficult for us to comprehend or imagine."

Campbell said that witnesses said that Barton was also upset about losses from investments in the stock market. Barton had quit his job with a chemical firm and had started trading stocks full time, becoming what is known in the securities business as a day trader.

Day traders usually buy and sell small quantities of extremely volatile stocks, usually in technology firms, hoping to make quick profits on stakes held for less than one full day and sometimes as little as a couple of minutes. Many day traders borrow money on margin to increase the number of shares they can buy and sell. It is a risky way to invest, and most day traders give up after losing the money they invest.

Harvey Houtkin, head of All-Tech, said in a telephone interview from the firm's Montvale, N.J., headquarters, that Barton had been a trader at the firm for about a year and had been a member of partnerships that had lost money. Such partnerships, a target of securities regulators, are often used to increase the amount of money their members can wager on stocks.

Like most day traders, Barton had experienced sharp swings in fortune but Houtkin said they weren't unusual. Although the technology stocks popular among day traders dropped sharply last week and today, Houtkin said Barton had not traded at All-Tech since April.

"Nothing can justify this type of behavior," Houtkin said.

In an interview with The Washington Post earlier this year, Houtkin warned that day-trading required training and compared it to firing a high-powered rifle. "If you use it right, you will be able to hunt effectively and bring down nice buck. But if you don't know what you're doing, you'll probably blow your head off," Houtkin said then.

Houtkin said today that "My emotions are so screwed up right now. This is such a tragedy . ... This is your worst nightmare."

A spokesman for Momentum Securities said they did not know of any connection with Barton.

Vice President Gore telephoned Atlanta's mayor, Campbell, and offered federal assistance in trying to catch Barton. Campbell released a photo of Barton's driver's license, showing a full-cheeked man with a receding hairline dressed in a jacket and tie.

Witnesses gave different accounts today of what Barton was wearing, Atlanta police said. Some said he was wearing khaki pants and an orange shirt. Others said he was wearing shorts.

The wounded were taken to three different hospitals. Seven victims arrived at Grady Hospital. One, a woman who was shot in the back, was released from the hospital after treatment. Of the six others, all of them men, three remained in critical condition and three were in stable condition.

A St. Joseph's hospital spokesman said two victims had arrived, both in critical condition. The mayor said that three others wounded were taken to Northside Hospital.

Witnesses were taken aside by police investigating the case. At the scene, Cabrer, the tollway authority worker, said he heard witnesses say, "He used to come in, he was always a funny guy. So we thought he was joking with us."

Mufson reported from Washington, August from Atlanta.