The lawyer representing a Dutch man detained on suspicion of involvement in terrorism says he is actually an amateur sleuth who tries to track down extremists via social media.

Karianne Bal said Friday that her client is being held on suspicion of inciting a possible terror crime, but that he was actually engaged in his "unusual hobby" of trying to unmask extremists.

Bal says her client has previously been in contact with the main Dutch spy agency about his detective work. The agency declined comment.

Prosecution spokeswoman Jeichien de Graaff says that investigations continue into the man's "actions, his motives and his contacts."

The suspect was detained 24 August following a terror threat that forced the cancellation of a concert in Rotterdam by Los Angeles rock band Allah-Las.

Allah-Las said in an interview last month that they often receive threats from people offended by the use of the word 'Allah' in the group name.

"We get emails from Muslims, here in the US and around the world, saying they're offended," the group told The Guardian.

They added that causing offence "absolutely wasn't our intention" and that they chose the name because they wanted something "holy sounding."

The investigation continues.