It was an attack that shocked two ­nations — a 26-year-old Brazilian woman living in Zurich claimed to have been attacked by racist skinheads who carved the initials of Switzerland's main rightwing party into her body and caused her to miscarry her unborn twins.

When news reached Brazil earlier this week, the foreign minister, Celso Amorim, condemned the assault as "grave" and "shocking" and called in the Swiss envoy. The president, Luiz Inácio Lula da Silva, said: "We want people to respect Brazilians overseas in the same way that we respect them here. We cannot accept and stay silent faced with such a level of violence against a Brazilian woman overseas."

The story was splashed across newspapers under headlines such as The Marks of Intolerance, and led most television bulletins.

But today, Paula Oliviera's story appeared to be unravelling. Swiss authorities said she was not pregnant when the attack was alleged to have happened. And according to medical experts, she may well have cut herself. The head of Zurich University's forensic medicine department, Walter Baer, said that "any experienced forensic doctor would not hesitate to assume that this was a case of self-infliction".

Chastened by the sudden twist to the episode, Brazilian officials were silent. A member of Lula's press office said the presidency would not make any official comment on the case, adding that it appeared to have taken on "new dimensions". A spokesman for Brazil's foreign ministry said it would "wait for the end of the investigation before making new statements".

Oliviera, a 26-year-old lawyer, told Zurich police she was attacked by three skinheads, one with a Nazi symbol tattooed on the back of his head, outside a Zurich train station on Monday. Pictures have shown her stomach and legs scarred with the initials of the rightwing Swiss People's party (SVP).

The woman's family told the Brazilian media she had been speaking Portuguese outside Stettbach train station shortly before the attack.

All week, Oliviera has been interviewed from her hospital bed by police and forensic experts. Her fiance said she was suffering from nightmares. "She woke up sweating and screaming," said Marco Trepp.

Police have appealed for witnesses and say they have spoken to three men who were near the scene of the alleged attack but that no one was arrested.

The SVP won 29% of the vote in the last election and is known for its anti-immigration stance.

During the 2007 election, its poster showed white sheep kicking a black sheep out of Switzerland. It caused international outrage.

A spokesman for the party, Oskar Freysinger, condemned the alleged attack and denied any involvement. "If that really was someone from our party, we wouldn't hesitate for a second. That person would be immediately kicked out," he said.