Take immediate action. Don't let the situation rest while a leisurely investigation takes place. If necessary, suspend the individual with pay pending the investigation. Employers should be cognizant of the potential problems of workplace violence and must practice prevention even prior to hiring. Be careful who you hire.
Companies do a poor job of checking applicants' backgrounds.
It should be done. Some employers complain that making the phone call to the previous employer is fruitless because all they get is the proverbial name, rank and serial number. That's probably because they speak to the HR Manager who is trained to give out minimal information. Instead, ask the applicant for permission to call his or her former supervisor. You will get much more information out of a supervisor than you will out of calling the HR department.