"The unemployed suspect, who lived alone, was apparently intensely active on notorious internet platforms."

High alert

Germany remains on high alert after the video postings, with security at airports and train stations stepped up over the last seven days.

Germany remains on high alert ahead of Sunday's federal elections [AFP]
"We've raised security to a level appropriate for the scenario and it will stay at this level for the time being," Stefan Paris, an interior ministry spokesman said.

The first two videos showed an al-Qaeda messenger, identified as Bekkay Harrach, a German-Moroccan, saying Germany will pay a price if voters back a government that supports deployment of troops in Afghanistan.

In the third video, released on Thursday, a masked man believed to be Harrach calls on Muslims in Germany to take part in jihad, or holy war.

Only one party of the five in Germany's lower house of parliament is calling for a withdrawal of the 4,200 German troops serving with Nato forces in Afghanistan.

Polls show Angela Merkel, Germany's chancellor, is likely to win Sunday's vote.