A signal controller is at the heart of the German investigation into a Bavarian train disaster which claimed 10 lives, media reports say.
German police have so far rejected as speculation the reports that the controller turned off an automatic safety system shortly before the crash.
But unnamed sources have told German media that a human signalling error may have led to the two passenger trains colliding on a single track on Tuesday.
Controllers ensure trains run safely.
Unanswered questions
In focus: Bavaria's railways
The trains crashed head-on while both were travelling at about 100km/h (62mph) east of Bad Aibling, a spa town about 60km south-east of Munich.
A teenage girl as well as both train drivers and two train guards were among those killed.
Eighteen people were seriously hurt.