Two German prison guards face a criminal investigation after they allowed a convicted rapist to escape during a visit to a beer hall.

German news agency dpa reports that the Aachen penitentiary where the guards worked alleges they failed in their duty.

A spokesman for the justice ministry in North Rhine-Westphalia state says 58-year-old Peter Breidenbach was on a supervised day trip in Cologne when he gave his guards the slip during a stop at a popular brewery. Accounts differ on whether Breidenbach was allowed to go to the toilet alone.

Spokesman Marcus Strunk said Friday the incident was "unacceptable" and would be fully investigated.

Breidenbach was sentenced to nine years in prison in 1991 but remanded in prison afterward because a court deemed him to be particularly dangerous.