Who is responsible if part of house collapses after selling

I am asking this question as an outsider with no knowledge of intricacies of law. Suppose Mr. A purchases an apartment on tenth floor of a residential flat and he sells it to Mr. B. After few months the iron grill on the balcony of his apartment falls on the ground and one innocent person gets killed. The iron grill was part of the apartment sold by the builder to Mr. A but he also made some alterations to it at his own expense. In this case who will be punished by the court in due course? Mr A or Mr B or the builder or the worker(s) who actually carried out the iron grill work?