The U.S. has strong indications that chemical weapons were not used in the attack in Syria’s Aleppo province on Tuesday, a U.S. administration official said Thursday.

While officials won’t entirely rule out the possibility, the official said that additional intelligence-gathering in recent days has led the U.S. to believe more strongly that it was not a weaponized chemical attack. The official wasn’t authorized to speak publicly about the matter because it involved intelligence gathering and spoke on condition of anonymity.

The U.S. and allies have been looking into allegations by the Syrian regime that rebels carried out a chemical weapons attack on Khan al-Assal village in northern Aleppo province. The rebels have blamed regime forces.