Ruseva had said she gave birth to a baby girl four years ago in Greece while working there as an olive picker but gave the child away because she was too poor to care for her. She since has had two more children after Maria.
Maria has been in a charity's care since authorities raided a settlement of Roma, also known as Gypsies, in Greece last week and found she was not related to the Greek Roma couple she was living with.
Her discovery triggered a global search for her parents, fears of possible child trafficking and interest from authorities dealing with missing children cases in Poland, France, the United States and elsewhere.
Human rights groups also have raised concerns that the news coverage about Maria and the actions taken by authorities were fueling racist sentiment against the European Union's Gypsy minority, who number around 6 million.
The Bulgarian prosecutor's office and Greek authorities were "seeking clarification on whether the mother agreed to sell the child," the Interior Ministry said in a statement.
It wasn't clear if Maria had been told who her real parents are. The Greek charity Smile of the Child, which has been looking after her, would not comment on the case.
Social services director Kaneva, after visiting the Ruseva home, said the child would likely be returned to Bulgaria for adoption.
"Maria will first be brought to a family crisis center, and after they (authorities) will seek for suitable foster family," she said.
The Rusevs and their other children live in a dilapidated, mud-floored house outside the remote Bulgarian town of Nikolaevo, 280 kilometers (175 miles) east of the capital, Sofia.
The Roma quarter in the town houses some 2,000 people. Most are jobless, living in extreme poverty, trying to stay warm in shabby houses. Children played Friday in mud-covered streets as pigs, cats and hens ambled by.
Minka Ruseva, a 14-year-old who is one of the Rusev family's children, stood in front of their dilapidated two-room house. Minka said she saw pictures of Maria on TV and thought she was her sister.
"I like her very much. She looks very much like me, and I want her back home. We will take care of her, and I can help my mother," she said.
Stoyan Todorov, a neighbor of the Rusevs, complained of the hardships that he and his family face every day. He said Bulgarian authorities do not care about helping the Roma and come "only on the eve of elections, hoping to get our votes."
"Look how we are living in total misery," he continued. "Years ago, a man was murdered in our neighborhood and nobody paid attention. Now there are crowds of concerned people here because of one girl."
As he spoke, he pointed at the scores of reporters from across Europe who had descended on the remote area.
"The truth is that we do not have the money to look after our kids," Todorov said.
Greek officials, fearing that Maria's 2009 birth record contained false information, have ordered a nationwide check of all Greek birth records in the last six years to ferret out welfare fraud or other irregularities.
The Greek Roma couple, now in pre-trial detention, have been charged with allegedly abducting Maria and committing document fraud. They told authorities they had received Maria after an informal adoption and their lawyer said Friday they planned to seek legal custody of the girl.
Under Greek law, child abduction charges can include cases where a minor is voluntarily given away by its parents.
"We are very, very happy with this outcome, because we have proved what we said from the outset. ... The adoption, as it happened, was not of a legal nature but it was not abduction," the Greek couple's lawyer, Costas Katsavos, told The Associated Press.
"Now, as the birth mother has been found, we will ask to gain through legitimate processes custody of little Maria, whom the family truly sees as its own child."
At the Gypsy camp in Farsala, central Greece, where Maria was found, residents said the couple had been vindicated.
"They are saying the woman stole the girl. She didn't steal her. The Bulgarian gave the child to her. ... We've had Maria here for five years," neighbor Christina Pavlos said.
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Paphitis reported from Athens, Greece. Konstantin Testorides in Skopje, Macedonia; Derek Gatopoulos in Athens, Greece; and AP Television staff in Farsala, central Greece contributed.
