The security company USIS is facing a litany of questions from Congress following the Navy Yard mass shooting. USIS is the federal contractor that vetted both Edward Snowden and the suspected Navy Yard shooter, Aaron Alexis. There are a lot of private companies that do background checks on employees, but that is just the first step in preventing violence in the workplace.

When mass shootings happen in the workplace, whether it’s a government or private employer the question of liability is always raised.

“One of the key issues of liability has to do with what did you know, when did you know it and what did you do about it,” says Dr. Larry Barton, an instructor of threat assessment at the FBI.

In the case of Navy Yard shooting, the government knew that Alexis had an arrest record, a history of mental illness and his coworkers reportedly said he held grudges over very minor issues.

These should have been warning signs. To prevent violence employers have to go beyond just background checks says Dan Murphy, the founder of the company Violence Prevention Strategies.

“There must be an employee, as well as supervisory and management, awareness as to what warning signs are indicators of violence or violent activity,” says Murphy.

The next step is to provide help for those employees who show signs of mental instability.

“The vast majority of people who are involved in these cases are basically good people who have had life issues, professional issues, personal issues and are on a downward spiral,” he says.  

Murphy says that if warning signs lead to intervention, many of these shootings can be prevented.