Although investigators do not have any specific evidence that Fiona was abducted, extensive searches through the 26-hectare Montjuzet park where she disappeared on Sunday evening have proved fruitless.
For that reason, police have opened a formal investigation into “kidnapping and false imprisonment,”
“By deduction, our primary theory…is that she could have been kidnapped and left the park with an adult,” local prosecutor Pierre Sennes told French media on Tuesday.
Despite elevating the line of inquiry to abduction, Sennes said the police's kidnap alert procedure had not been triggered.
This is because such an alert can only be raised where there is positive evidence that an abduction has taken place, or a witness statement to that effect.
“I think about everything and nothing...This is very hard."
Meanwhile, details have emerged about the troubled family life of the little girl, named only as Fiona.
Forensic police scoured the home of Cecille, Fiona's mother, who was questioned as a witness for six hours on Monday.
Furthermore, Cecille's biological father, as well as her current boyfriend – both of whom have criminal records – have been questioned as witnesses by police investigators.
For his part, Fiona's biological father was said to be “living in fear and fearing the worst,” his friend told Europe 1 radio.
“He is all over the place and is very upset. He's been searching everywhere, to see if his girl is out for a walk,” the friend added.
Fiona's mother told reporters she was “shattered,” since her daughter's disappearance. “I think about everything and nothing in particular. It's been two days. This is very hard, psychologically,” she added.
At the time of Fiona's disappearance she had dozed off and when she woke up, she was unable to find her daughter.
Fiona is "light skinned," has blonde hair, stands around 1.10 metres tall and at the time of her disappearance was wearing a black tracksuit which had the white logo "Hello Kitty" written on it.
She was also wearing black trainers and a gold chain around her neck, police say.
On Sunday evening and Monday police carried out an extensive search of the park, but to no avail.
Scores of soldiers, firemen and police reinforcements have been drafted in to join the search. Throughout Sunday night a helicopter, fitted with a special thermal imaging camera, circled above the city looking for any sign of her presence.
Police are calling for anyone who was in the park on Sunday, or who may have taken photos or videos on their mobile phones to contact 07 77 97 08 59 or 04 63 05 22 21.