Beijing’s national security office in Hong Kong has carried out its first known joint operation with city officials, marking an expansion of China’s direct law enforcement in the financial hub.

Local authorities on Thursday night announced they were investigating a case of alleged foreign collusion with China’s Office for Safeguarding National Security. The office interviewed six people after requesting the support of Hong Kong police using new powers granted last month, according to a statement.

Hong Kong security officials searched the homes of six suspects, as well as the office of an organization, and seized evidence including bank documents and devices. The individuals were also required to surrender their travel documents.