Police and MI5, Britain's domestic counter-intelligence and security agency, foiled what appeared to be the first ISIS-linked terrorist plot on U.K. soil, arresting four men in dramatic raids in by armed officers in London during which one suspect was Tasered by police. Authorities feared the men were in early stages of planning a "significant" attack, and security officials reportedly were investigating whether the four wanted to follow the ISIS playbook and execute someone on a British street. Investigators believe one of the men has links to Syria and ISIS. Authorities also said one of the group may have had access to weapons. Scotland Yard announced that the men, two of them 20 and two 21, were being held on suspicion related to the "commission, preparation or investigation of terrorism," in what London's police commissioner, Bernard Hogan-Howe, called "quite a serious case." Police were searching a number of homes and vehicles across London. The arrests come about two weeks after police arrested 10 other men in the U.K. on suspicion of terrorism offenses. Hogan-Howe told BBC radio many recent arrests are linked "back to Syria and Iraq." Concerns about ISIS-related attacks in Britain itself have grown since the country joined airstrikes by the U.S.-led coalition.