The Germans have asked Sweden to interrogate a 28-year-old man from Västerås, who was in contact with Krekar in 2010.
"The German police have requested that we interrogate him, and they want it done in a court of law," Prosecutor Ronnie Jakobsson told the magazine.
Although the man is not suspected of any crime, German police believe that he has information about an Islamist terror network they suspect Krekar and another nine people were seeking to establish.
According to the German investigation, the 28-year-old was in contact with other members of the network via Paltalk, a US-based chat service in 2010.
He also wrote articles for jihadist internet sites administered by Krekar's group, and had direct contact with Krekar on at least two occasions.
Krekar, born Najmuddin Faraj Ahmad, came to Norway in 1991 as a refugee from the Kurdish region of Northern Iraq. In 2001, he co-founded Ansar-al-Islam, the extremist Islamist group behind a wave of bombings in Northern Iraq after the 2003 invasion.
Krekar is currently serving a two-year sentence for making death threats against Conservative Party leader Erna Solberg, and against three Kurdish men.
He was acquitted of a more serious charge of "incitement to terrorism" by Norway's Court of Appeal in December 2012.