Is rejection possible even after the agency has informed that the interview was successful?

Saying that the interview was a success does not avails for the job. Do you know how many other people are being interviewed for that same position. Or it was just you??

If you were the only one then the offer should come soon. But if you are not the only one then you will have to wait.

In my case there were several interviews and they lasted about 2 months to make the offer. Although some people hast got the "offer" in 1-3 weeks time.

Once again it will depend on the situation, the position and how many persons are applying to that post.

Regards!!