Bulgarian police have identified a couple they suspect are the natural parents of a blonde girl found in a Roma camp in Greece.
Prosecutors are investigating the woman for selling her child, officials said this afternoon.
Last week's discovery of the girl, known simply as Maria, sparked a global search for her real parents after DNA tests showed the Roma couple she was with were not her blood relatives.
Bulgarian police questioned Sashka Ruseva, 38 and her husband, Atanas Rusev, 36, today in the southern town of Nikolaevo.
The couple are also Roma.
"The prosecutors' office has opened a pre-trial investigation against SR for agreeing to sell her child on an undisclosed date in 2009 in Greece," the regional prosecutor's office in the southern town of Kazanlak said in a statement.
"The probe is opened following checks linked to the female child with the name Maria in Greece," it said.
Ms Ruseva told reporters she had given birth to a girl in Greece in 2009 and had left her there as her family needed to go back to Bulgaria and had no means to support the child.
"I do not know whether she is mine or not. We had a child. We left it in Greece as I had nothing to feed her," she said. "I did not take any money."
Prosecutors said they would check Ms Ruseva's travel movements and carry out DNA tests to establish whether she was indeed the biological mother of Maria.
Dozens of journalists have flocked to the poor Roma camp in Nikolaevo, where the Roma couple, their ten children and another relative live in extreme poverty in a shabby two-room house.
Police said in a statement the woman had told them she had recognised the Greek Roma couple with whom Maria was found as the people she had left her child with, after seeing them on TV yesterday.
If found guilty of selling her child, Ms Ruseva faces up to six years in jail and a fine of up to 15,000 levs (€7,700).
The Roma couple in Greece have been detained pending trial on charges of abducting a minor.
They deny the accusations, saying the girl's biological mother gave her up willingly because she could not raise her.
Ms Ruseva told reporters she left her baby to a woman in Greece four years ago when she and her husband worked there but needed to go back to Bulgaria to take care of their other children.
She said she was not completely certain that Maria was the child she had left behind, but told police the girl resembled some of her other children.
If DNA tests proved she was, she said she would like to take her back.
There are an estimated 10 million Roma living across Europe, and they are one of its oldest minorities.
The Council of Europe, which monitors human rights, says they are also the most discriminated-against.
Greece and Bulgaria have agreed to crack down on an illicit baby trade between the two countries, Greek public order minister Nikos Dendias and Bulgarian Deputy Prime Minister Tsvetlin Yovchev said in Athens this afternoon.