Internal audit ..who should do it ?

Respected Members,

Can you please guide as to who can be appointed as Internal Auditor ? sec 227(4A) provides a reference on the requirement of having Internal Audit systems in place, But on plain reading of the clause ,i have not come across any provision which defines Internal auditor i,.e who can become Internal Auditor. & i am assuming that its mandatory to have Internal audit system within the company ( someone performing this role from within the company)

To summarize my query: Can an employee become an Internal Auditor ? or a CA firm has to be iven a mandate for Internal Audit exercise ?

Pls guide

thanks in advance