British police and intelligence agencies arrested three more suspects Wednesday in connection with the Manchester suicide bombing and moved quickly to secure key sites across the country, including Buckingham Palace and the British Parliament at Westminster.
Home Secretary Amber Rudd said the bomber, identified as British-born Libyan Salman Abedi, "likely" did not act alone when he killed 22 people and wounded dozens at an Ariana Grande concert Monday night in Manchester.
She said he had been known to security forces "up to a point."
Police said three men were arrested Wednesday in south Manchester, where a day earlier a 23-year-old man was also arrested and a number of homes were searched.
Britain raised its threat level from terrorism to "critical" after an emergency government meeting late Tuesday amid concerns that the 22-year-old Abedi may have accomplices who are planning another attack.
British soldiers have been deployed in place of police officers to guard high-profile sites such as Buckingham Palace and Parliament.