MADRID — The Spanish woman at the heart of a nationwide scandal involving babies who were stolen and trafficked over a four-decade period said on Thursday that she had discovered she was the victim of an illegal adoption rather than an abduction.

Almost a decade ago, Inés Madrigal became the first person to be recognized by a Spanish court as having been a “stolen baby,” taken by traffickers. Her courtroom victory prompted many other people to start lawsuits in search of their biological mothers, in the belief that they, too, had been stolen at birth.

But at a news conference on Thursday, Ms. Madrigal said that she had established that her biological mother had given her up for adoption with the help of a doctor who falsified the documentation.

Ms. Madrigal, now 50, said she had used a DNA databank to locate a biological cousin in the United States and then reunite with several previously unknown biological siblings, who told her about the adoption.