@ssara

The main purpose of investigation is to confirm that a candidate has clean criminal history or is not involved in any unlawful activity. They also verify whether any kind of FIR has been registered against the candidate.
They visit house of the candidate to confirm that the address mentioned by him/her on his documents is correct and that he/she is actually a temporary/permanent resident of that house.
They usually don't visit educational institution / employer because it is not their responsibility to verify candidate's educational documents. However, if they are in any doubt and need to furthr probe into candidate's character etc, they may visit his her educational institution / employer office for detail of his/her activities over there.

If someone is involved in a criminal activity and he/she doesn't mention it on his security performa, there is a possibilty that the candidate might have to face consquences incase they find it out somehow. But I am not sure how severe these may be or whether it may cause disqualification or not.

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