Carla Espinosa-Alvarez's mother had threatened to take the girl from her foster home, police said.
On Monday, one day after the 3-year-old was reportedly kidnapped at gunpoint, mother and daughter turned up safely in Mexico.
But the discovery of the girl only raised more questions about the Sunday night abduction from her North Las Vegas foster home.
Sgt. Tim Bedwell, a North Las Vegas police spokesman, said detectives were investigating the circumstances of the girl's disappearance.
"Until we sort out what truly happened, we won't know what the next step is," he said.
The girl's foster mother told police that a masked couple tricked their way into her home near Commerce Street and Lone Mountain Road to take the girl.
Once inside the two-story house on the 100 block of Rosa Rosales Court, the masked man held the foster mother at gunpoint while the masked woman searched the house, found Carla and carried her away, police said.
The foster mother was a licensed foster parent with no connections to the girl's family, said Christine Skorupski, spokeswoman for the Clark County Department of Family Services.
The agency had been caring for the girl since October, and Carla had been at the home for seven months. Her mother, Maria Espinosa-Alvarez, is a Mexican national who had threatened to take her away, police said.
Espinosa-Alvarez had left her daughter with friends in Las Vegas and went to Mexico to attend to some affairs. But when she tried to return to Las Vegas, she was denied entry into the country, sources said.
Family Services eventually took the girl from the mother's friends because they were not legal guardians. No charges were filed against the mother or the friends, and the girl was placed in foster care.
Espinosa-Alvarez had been working with a lawyer and the Mexican consulate in Las Vegas since November to resolve the custody issue in Family Court, consulate official Octavio Perales Sanchez said. Carla has no known father, he said.
Skorupski said the Family Court case was ongoing.
"We were working with Carla's mother to see if we could reunify her with her daughter," she said.
But the case was short-circuited with Sunday's apparent kidnapping.
On Monday, Espinosa-Alvarez went to a consulate office in Mexicali, a border town about 350 miles south of Las Vegas, with her daughter, Perales Sanchez said.
Whatever happens next would be handled by federal agencies on both sides of the border, he said.
"Now it's in Mexico, so it's really out of our hands," he said.
Said Skorupski: "It's now an international law issue."
Anyone with information about the case can call the National Center for Missing and Exploited Children at 1-800-THE-LOST.