In the past three weeks, at least 13 people – including three in San Antonio – have died nationwide as a result of shootings in the workplace.

"Workplace violence is very common," said Pamela Porter, trauma specialist and consultant with Crisis Management International.

While on-the-job violence has been on the decline for the last decade, homicide still ranks as the second-leading cause of death at work, according to the Occupational Safety and Health Administration.

The San Antonio shooting Wednesday at Century 21 shows how suddenly and unexpectedly violence can occur at work. In a violent outburst, Ron G. Thomas, a real-estate agent and father of two, killed two co-workers with a .357 Magnum handgun and critically injured a third. He fled the scene and later committed suicide.

The San Antonio incident is not an isolated one. On July 8, a gunman killed five people and then killed himself at a Lockheed Martin aircraft parts plant near Meridian, Miss.

And on July 2, a factory worker fatally shot three colleagues and wounded five others at the Modine Manufacturing Co. near Jefferson City, Mo. The gunman later shot himself to death.

These cases show how workplace violence poses a serious threat to employees and a serious problem for employers, especially small businesses that might not have policies in place to deal with aggressive employees. The reasons for workplace violence vary, Porter said, but often involve employment termination, mental issues, financial and domestic problems, and alcohol or drug abuse.

"The common thread that motivates someone to resort to violence in the workplace is that they feel they have been treated unfairly," Porter said.

Plant closings, layoffs and a downturn in the economy can also be triggers, said Michael McIntyre, an industrial psychologist at the University of Tennessee.

"There are always aggressive people out there who are going to be triggered by something," McIntyre said. "It's always some combination of stuff.

"I'm not going to absolve the shooter by saying the company mistreated them. There's always something else going on with these folks."

Traits to watch out for are people exhibiting aggressive tendencies and paranoia who constantly blame others while focusing on power, he said.

"It's not hostility – it's extreme anger," he said. "We're not talking about assertiveness."

People who pose a threat at work generally exhibit some change in behavior. The classic characteristics include the start of problems at work that shows up in their productivity or absenteeism, Porter said.

"They might seem stressed, grumpy and they might make off-handed comments about getting even," he said. A reassignment of a work position can trigger violence. Some employees view a reassignment as a personal attack. Other signs to watch out for are gestures, including someone using their fingers to point, as with a gun, at someone. "People tend to dismiss it. They say, 'That's just they way he is,'" Porter said. "People don't want to think violence is going to come into their workplace."

But it does. Workplace violence in the United States involves 1.5 million incidents each year, said Paul M. Viollis, senior managing director at Citigate Global Intelligence & Security, based in New York.

"Workplace violence is the No. 1 recognized hazard in the United States today," Viollis said. "It's never spontaneous and it is always avoidable."

Employers should take care to establish a violence avoidance policy and to train all employees in dealing with threats in the workplace, Viollis said.

"The tragedy about this – aside from loss of life, the assault and the emotional trauma – is that this doesn't need to happen," Viollis said.

Companies have to deal with it just as they deal with sexual harassment and other issues, he said. "The liability of not having a workplace policy in place is just way too high for employers," Viollis said. If an employee makes a threat, his or her employer should take that very seriously, said Jon Miller, employment law specialist for California-based law firm Berger Kahn.

In case of a firing, employers also should make sure the employee no longer has access to the site by taking away keys, computer codes or parking access.

And an employer should always alert security, letting that department know a terminated employee shouldn't be permitted on site without the approval of an office manager.

"Firing for anyone is a traumatic event," Miller said.

"For most people, while it is traumatic, they can deal with it. For a small percentage of the population, they can't deal with it."

The least expensive and most successful way to diffuse a threat of violence is communication, Porter said. "If you can't talk with the person, you need to get someone who can," she said.