Police said Alisa Maier was playing with her 5-year-old brother outside their home in the town of Louisiana Monday night when a man pulled up in a black four-door passenger car and took her away, KSDK reported.
The only witness to the alleged abduction was the girl's brother, who told police he saw his sister get into the car with a man he did not know, Louisiana Police Chief Richard Hughes said at a news conference Tuesday.
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Police have only a vague description of the alleged abductor. He is possibly a white male with dark hair in his late teens or early 20s.
Authorities have issued an Amber Alert that describes Maier as 3-feet tall and 40 pounds with brown hair and brown eyes. She was wearing a white T-shirt and blue jean shorts when she disappeared.
Alisa's great grandmother, Mary Foiles, said the girl's parents were inside the home when the alleged abduction occurred.
Police searched a small creek behind the girl's home Tuesday morning for clues. Dozens of volunteers passed out fliers.
Hughes said a weekend carnival had brought more strangers than usual to the town, which is located about 70 miles northwest of St. Louis.