A second suspect was arrested late on Saturday in connection to Friday’s subway attack in London. Police announced on Sunday that a 21-year-old man was detained in Hounslow in west London and is being held under the Terrorism Act. He has not been charged or identified yet. Two men have now been held in custody for their possible connection to the bombing attack on a rush-hour subway train Friday morning that injured 29 people in London. An 18-year-old man was arrested Saturday in the departure area of the port of Dover, where ferries leave for France. The two arrests indicate police and security services believe the attack at the Parsons Green station was part of a coordinated plot, not the act of a single person. "We are still pursing numerous lines of inquiry and at a great pace," counter-terrorism coordinator Neil Basu of the London police said late Saturday. Prime Minister Theresa May put Britain on its highest security level of “critical” late on Friday, meaning another attack might be imminent. Soldiers and armed police were deployed to strategic locations such as nuclear power plants. Police on Saturday launched a massive armed search in the southwestern London suburb of Sunbury. Neighbors were evacuated in a rush from the area and kept away for nearly 10 hours before they were allowed to return to their homes. The ISIS terrorist group Friday’s attack was carried out by one of its affiliated units. Intelligence officials say there is scant evidence the extremist group was behind the attack. The improvised explosive device placed on the subway train only partially detonated, limiting the number of injuries. Home Secretary Amber Rudd said the casualties would have been far higher if the bomb had fully detonated. Frustrated by the string of terrorist attacks in recent months, she said officials will have to work harder to make bomb components more difficult to obtain. Britain has endured four other attacks this year, which have killed a total of 36 people. The other attacks in London — near Parliament, on London Bridge and near a mosque in Finsbury Park in north London — used vehicles and knives to kill and wound.