Al-Barnawi and six other co-accused persons, including his second wife, are accused of kidnapping and killing a British national, Christopher McManus and other foreigners working in Nigeria in 2012.
Police in the German city of Munich have asked members of the public to submit any videos, photos or audio recordings from Friday's mass shooting.
Nine people were killed and 16 wounded, three of them critically, at a shopping mall by a lone gunman who later shot himself dead nearby.
