A 20-year-old woman who testified against the National Socialist Underground far-right extremist group at a closed-door hearing was found dead in her apartment on Sunday, police said.

She had given evidence at the beginning of March to a special NSU investigation committee in the southwestern state of Baden-Württemberg, where right-wing extremists are alleged to have killed a policewoman in 2007.

The witness was the ex-girlfriend of a man believed to have had connections to the NSU, known only as Florian H. He was a former neo-Nazi at the time of his death, which occurred under mysterious circumstances in autumn 2013.

Florian H. was found burned to death in a car on the same day he was due to be questioned by police, as he was thought to have known who had killed policewoman Michèle Kiesewetter.

Kiesewetter is believed to have been a victim of the NSU, which is also alleged to have carried out the killings of nine immigrants and a series of bombings and bank robberies.

Beate Zschäpe (pictured above), the only surviving alleged member of the NSU, is currently on trial in Munich.

The 20-year-old witness found dead on Sunday, had testified in a hearing closed to the public, as she said she felt "threatened." The police said she died in her apartment in the southwestern city of Karlsruhe after suffering a seizure.

A police spokesman said there was no evidence of foul play, but an investigation was being launched into her death because of her role as a witness against the NSU.

Wolfgang Drexler, the Social Democrat (SPD) vice president of the Baden-Württemberg parliament and head of the NSU-investigation committee, urged the public not to speculate about the nature of her death, saying "we must await the results" of the autopsy.

es/lw (dpa, AFP)