The U.N. office in Damascus said in a statement Friday that its team will write up a report "it hopes will be ready by late October."
It was previously thought the U.N. probe would only look into three locations in Syria.
The United States says the Aug. 21 attack in the Damascus suburb of Ghouta killed 1,429 people. Ghouta has been the focus of the U.N. team so far.
The northern town of Khan al-Assal is also on the list. President Bashar Assad's government and rebels have traded accusations of chemical weapons use in an attack there on March 19.
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