Perhaps a visual see and avoid maneuver then?
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Assuming the "sudden turn" is what we see on the picture above, this is first 30 degrees for 25 NM.then another for 30 NM . Hardly collision avoidance turns. if a/c was not in contact with ATC or gave unsatisfactory answers , good reason to have a look and intercept I would say.
For those using FR24 more intensely , is the blue dotted line the original flight plan route , based on the actual PLN or just random ?
TCAS : what peekay4 says is absolutely correct technically , except that international Interception procedures require the first interceptor ( the one that comes close to the target ) to switch off his transponder just to avoid TCAS RAs from the aircraft intercepted, vertical manoeuvres that could be wrongly interpreted by the interceptors.