MCALLEN, Tex. — The bodies of what appeared to be a migrant woman in her 20s and three children — two infants and a toddler — were found Sunday night near the edge of the Rio Grande outside the South Texas city of McAllen, the authorities said.

Migrant deaths happen with grim regularity along parts of America’s southwestern border, largely when adults and unaccompanied teenagers succumb to harsh desert conditions or a lack of water, and die of dehydration, heat stroke or hypothermia. The discovery on Sunday was unusual — it is rare for officials to discover dead migrant children on the American side of the border, and rarer still for the bodies of three children to be found together.

“Most of the time, we usually find either adults or teenagers, but this is the first time we’ve actually found infants and toddlers, and it is pretty shocking for us,” said the Hidalgo County sheriff, J.E. Guerra, who broke the news of the discovery on Twitter late Sunday night.

Officials said there were as yet no signs of foul play, and that the four may have died from dehydration and heat exposure. The bodies appeared to be those of undocumented immigrants, but neither their identities nor their country of origin had been determined on Monday.