The Metropolitan Police Service arrested four men who are 20 and 21 years old at addresses across London, police said in a statement. The operation follows the arrests on Sept. 25 of nine men on suspicion of belonging to and supporting a banned organization and encouraging terrorism.

“A number of residential addresses and vehicles are being searched by specialist officers in west and central London as part of the investigation,” the MPS said today.

Britain joined the U.S.-led coalition of European and Arab allies carrying out military strikes against Islamic State positions in Iraq and Syria. The U.K. in August raised its terror threat to “severe,” the second-highest level, due to new intelligence, with Prime Minister David Cameron saying the battle against Islamic extremism is a “generational struggle” which will probably last decades.

Police in August called on religious leaders, members of the public and family to help identify home-grown terrorists to prevent the U.K. from becoming an export hub for jihadist violence. The first six months of the year have seen a total of 69 terror-related arrests, with the biggest growth in offenses connected to Syria occurring in London, the MPS said at the time.

Today’s arrests and searches are part of a continuing investigation into Islamist-related terrorism, police said.