Germany's domestic intelligence agency (BfV) confirmed on Wednesday that a 51-year-old man - arrested on suspicion of espionage and planning an Islamist terrorist attack - "radicalized himself undetected."

"We are evidently dealing with a case in which a person radicalized himself undetected within his personal surroundings," Hans-Georg Maassen told the German News Agency (DPA)

The news follows reports in German magazine "Der Spiegel" on Tuesday that the man's family was not even aware he had converted to Islam in 2014.

The Spanish-born German national was arrested in the western German city of Dusseldorf after allegedly disclosing sensitive information to another user on an extremist chat site. He also allegedly gathered information on the times and details of raids against extremists.

'For Allah's purpose'

According to prosecutors, the former bank-worker was only hired by the BfV in April, where he joined a new task force, monitoring the ever-growing Salafist scene in Germany.

German daily "Die Welt" also reported that he had also boasted about having a plan in place to carry out an attack on his place of work, though according to authorities he did not appear to have yet taken any concrete steps into carrying it out.

The suspect allegedly wrote online that such an attack against "unbelievers" would be "for Allah's purpose."

"Like every intelligence agency, (the BfV) is the target of infiltration attempts by foreign services, extremists and terrorists, which is why we as a security agency have to be particularly vigilant with regards to intruders," Maassen said.

The father of four had been under surveillance for about four weeks before an arrest warrant was issued for him, resulting in his detention.

Germany's main opposition party, the Social Democrats (SPD), on Wednesday called for a full inquiry into how the mole was recruited despite screening measures. "This has revealed a possible gap in security," said Social Democrat Burkhard Lischka.

Germany has remained on high alert for possible "Islamic State"-inspired operations since last year's Paris terror attacks, as well as a series of others to hit the continent, including in Nice and Brussels. The jihadi militant group also claimed responsibility for two attacks carried out in southern Germany during the summer.

ksb/kl (AFP, dpa)