USA TODAY’s analysis found that the most common motivator behind a workplace killing is a fi ring, which preceded about 60 of the 224 fatal attacks. The second most likely trigger is an on-the-job argument, a fight, or disagreement.

But those aren’t the only catalysts. Workplace attackers are likely to turn violent after, or during, a disciplinary action such as being written up for tardiness or poor performance.

A killer will generally react within days or weeks of a triggering event such as a fi ring, although some employees have returned to kill years after being fi red. Others have hunted down co-workers in their homes.

Few show mercy, often killing co-workers even as they plead for their lives. They shoot victims in the back and kill them as they cower their heads under desks or in bathroom stalls. They tie some up first and make others lie on the floor before they’re executed.

The workplace killer will typically attack with a gun, but other methods have been used. A maintenance worker used a pitchfork to stab a co-worker in the face and chest, killing him. A handyman set fi re to a hotel where he worked, killing fifteen people. A manager was convicted of first-degree murder for spiking a water cooler with cyanide, which killed a secretary, who collapsed after taking a drink.

According to an analysis of 1997-2002 done by the Department of Labor for USA TODAY, workplace killers are most likely to work in the services industry, followed by the retail trade. In the cases analyzed, the youngest killer was 17; the oldest 73.

The average workplace attacker will kill two employees. Secretaries, security officers, customers and anyone else who gets in their way can fall victim. They may make hit lists, plan their escape route violent movies and in some cases, leave behind notes or videotapes to help explain their rationales. Some have been hospitalized for mental health issues or have been on psychiatric medications before they killed.

The attacks may cap a life of failure or loss. Workplace killers are loners; among those cases in which marital status could be determined, approximately 40% were married.