When Amanda Hawkins arrived at Peterson Regional Medical Center in Kerrville, Tex., her two daughters were in grave condition.

As Ms. Hawkins told it, she and the toddlers had been at nearby Flat Rock Lake when one of the girls, then the other, collapsed. She wondered aloud whether the children, who she said had been smelling flowers, had come into contact with something poisonous.

The girls died Thursday at a hospital in San Antonio. On Friday, the authorities in Kerrville, about 65 miles northwest of the city, painted a very different picture of events than Ms. Hawkins had described.

The Kerr County sheriff, Rusty Hierholzer, said Ms. Hawkins, 19, had intentionally left the girls — Brynn Hawkins, 1, and Addyson Overgard-Eddy, 2 — in her car for more than 15 hours while she was at a friend’s house.