Straubing, Germany  - A convicted sex killer took a prison's chief psychologist hostage in Germany and allegedly repeatedly raped her before surrendering to police early Wednesday, Bavarian officials said.

The state's justice minister, Beate Merk, said police were still investigating how the 51-year-old prisoner, who had faced his first sex charges at the age of 13, had obtained a knife and managed to tie up the woman in her office Tuesday evening.

He surrendered several hours later after armed police had surrounded the prison at Straubing, near Munich.