I have seen and heard this asked quite a few times recently.
The system used by the UN to investigate allegations of chemical weapons use began to be developed in 1987 during the Iran-Iraq war.
The UN General Assembly and UN Security Council specifically requested that the Secretary-General carry out investigations which would only 'ascertain the facts of the matter' and 'report the results' to the Member States and the Security Council.
The Secretary-General and his investigative teams were not given the authority to make judgements as to who was responsible for any confirmed use of chemical weapons.
UN General Assembly Resolution 42/37C states:
4. Requests the Secretary-General to carry out investigations in response to reports that may be brought to his attention by any Member State concerning the possible use of chemical and bacteriological (biological) or toxin weapons that may constitute a violation of the 1925 Geneva Protocol or other relevant rules of customary international law in order to ascertain the facts of the matter, and to report promptly the results of any such investigation to all Member States;
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UN Security Council Resolution 620 states:
2. Encourages the Secretary-General to carry out promptly investigations in response to allegations brought to his attention by any Member State concerning the possible use of chemical and bacteriological (biological) or toxic weapons that may constitute a violation of the 1925 Geneva Protocol or other relevant rules of customary international law, in order to ascertain the facts of the matter, and to report the results;
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During 1988 and 1989 the system which would be used to conduct such investigations began to take shape and progress on this was reported to the General Assembly in UN General Assembly Document A/44/561 '
CHEMICAL AND BACTERIOLOGICAL (BIOLOGICAL) WEAPONS Report of the Secretary-General'.
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