My flight BA 117 LHR JFK was cancelled yesterday (April 26). A post above indicated reason was TECY.

Will I need any proof of the reason in order for my EU261 claim to not be rejected? (We’re family of 4, flying on revenue Club Suites NBO- LHR- JFK, and got rebooked on a flight 36 hours later). How should I go about obtaining this? Asked at the desk and the agent didnt give any concrete reason.