All this week we've been investigating the threat of violence at work. So far we've looked at how to recognize the warning signs and what to do if it happens.

Today, we have some practical steps to keep yourself safe. CNN's Alina Cho reports for part three of our special series "When Co-Workers Kill."

For Johna Lovely, who lives in Presque Isle, Maine, news coverage last month of Annie Le’s murder, allegedly by a co-worker at Yale University, brought back painful memories of the day she lost her daughter. “It brought everything back,” Lovely said. “I just cried and cried.”

Erin Sperrey was killed by a co-worker on January 2, 2005 in Caribou, Maine.

Lovely’s youngest daughter, Erin Sperrey, was killed by a co-worker on January 2, 2005. Sperrey was a supervisor for a fast food restaurant in Caribou, Maine. She was working the overnight shift with one other employee – Christopher Shumway.

Shumway is now serving 45 years for beating Sperrey to death.

Nationwide, 517 people were murdered at work last year according to government statistics. And while that number is down 52 percent since 1994, an American College survey found things like bullying, harassment, and physical altercations are up.

Laurence Barton, who studies workplace violence at the American College, a nonprofit educational institution that trains financial services professionals, says that kind of violence is becoming epidemic.

“The call volume to human resource officers, to their EAP programs, to counselors is sky rocketing,” Barton says. “We are absolutely in a period right now of among the highest periods of threats at work in certainly recent memory.”

That doesn’t surprise Lovely and her daughter, Amanda. They’ve worked tirelessly since Erin Sperrey’s death to stop workplace violence. They’ve set up a fund in Erin’s name (erinsfund.org) and have traveled around Maine to convince companies to install panic buttons, connected to police departments, so employees in danger can get immediate help. They thought armed with Erin’s story it would be a cinch. They were wrong. They told us just eighteen companies out of hundreds agreed to install new security systems or educate their employees about workplace violence.

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Raymond Clark, the man accused in the murder of Yale grad student Annie Le, is back in court later today. That case is bringing the issue of violence at work back to the front burner.

So how do you know if you're safe in the office? Our Carol Costello has part two of our special series, "When Co-workers Kill."

Erin Sperrey was working as a supervisor at a Tim Horton's in Maine when she was killed by one of her staffers. CNN's Carol Costello interviews Erin's mother and sister about their efforts to promote workplace safety.