No matter how misconstrued, inaccurate, or even false the statements made by an angry parent, taxpayer, etc., it is almost always disastrous to interrupt them.  (See; "Logic Never Trumps Emotion").  People need time to ventilate and if you don't give them the time they need to fully ventilate that anger, they will become even more frustrated.  The rule of thumb is that it takes about 20 minutes for an angry person to thoroughly ventilate his/her anger.  If he/she pauses before then it is usually just to catch his/her breath.  And if you make the mistake of thinking the person is finished and try to answer the issues raised, that person will usually take off again.  So if the angry person pauses, it is usually best to ask open ended questions that encourage him/her to ventilate some more, e.g. "is there anything else I need to know?", "was anyone else there?", "exactly when and where did this happen?", etc.  Once the person has thoroughly ventilated, then and only then can you begin to solve the problem.