Generally you do a Root Cause Analysis post-event, not a Fault Tree Analysis.

Fault Trees are typically used to determine the probability of a bad thing (fault) occurring. This probability is used to populate a Risk Matrix (Probability vs Consequence). The Risk Matrix is used to decide whether the risk is low enough vs the consequence within a design. It the risk is too high, then more redundancy or layers of protection are likely to be needed.

Root Cause Analysis builds a tree of possible causes that will look much like the Fault Tree diagram but may include more human factors (was the part inspected before use, was the equipment maintained etc).

There is a lot of overlap of techniques within the Risk Engineering discipline. Fault Trees can be mapped to Reliability Block Diagrams (AND gates are equivalent to Parallel pathways, OR gates are equivalent to Series pathways for example).

The mention of Fault Tree Analysis being required post event seems a bit odd. It may just have been a confusion of terms in the heat of the moment.