How do they know the murder occurred there? Where was the body? How many employees are there? What kind of company?
If it's a super-secure, small, tech-based company, with like 15 employees, cameras everyplace, no one in or out without a biometric scan or something, it'd be fairly simple to determine if it's an employee. First, there are only 15, so find them all or not. If not, then try that. If you do, then there's a problem and you go to the security to figure it out first.
If they find someone in an out-of-the-way area of a big department store, during open hours, and the body could have been there over two different shifts, it's much harder to figure if it's an employee or a member of the public. Takes way longer to track down workers, much more complex if it isn't one, etc.