Border Patrol reported 283 migrant fatalities on the border in 2018. Human rights activists say the number is far higher as the remains of many migrants are never found and the agency's data does not include all deaths registered by local authorities.
The girl died in a remote desert area west of Lukeville, Arizona after her mother left her with other Indian migrants she was traveling with to go in search of water, a medical examiner and US Border Patrol said.
Clouds of the noxious gas could be seen wafting up from around the fence at the border, and one migrant picked up a canister and threw it back into US territory. US officials said the group had attacked agents with projectiles, but a Reuters witness did not see any migrants throwing rocks at US agents.
The US government's system for detaining migrants crossing the border is severely overtaxed. Authorities would not say how many children US Customs and Border Protection is now holding. But the country is seeing a sharp rise in families with children.