A police investigation is gaining speed into North Korean defector groups which have been sending anti-Pyongyang leaflets across the border.

Speaking to reporters Monday, Lee Yong-pyo, commissioner of the Seoul Metropolitan Police Agency, said they have booked two individuals related to the distribution of leaflets.

He said a legal review is under way in connection to a number of laws including the inter-Korean exchange and cooperation act, aviation safety act, public waters management act and the criminal code.

The agency has set up a 40-member task force to look into leaflet campaigns and other articles activist groups are sending to North Korea.

During an onsite inspection in Paju, Gyeonggi Province on Sunday, officers found and seized 20 hydrogen gas containers. They are now analyzing security camera footage to see by whom they were used.

Commissioner Lee vowed a thorough investigation saying it is a serious matter that concerns the safety of border town residents. The Unification Ministry earlier filed a complaint with the police against two defector groups on charges of violating domestic law.