Britain reduced its terrorism threat level a notch, from "critical" to "severe," on Saturday as authorities said they had made major progress in unraveling the plot behind the Manchester concert bombing.
Police said more arrests were expected, and security was tight across Britain as hundreds of thousands of people attended major soccer matches, concerts and other big holiday weekend events.
Police on Saturday made two more arrests in Manchester on suspicion of terrorism offenses, bringing the number of suspects in custody to 11. All are men, aged between 18 and 44. In addition, Manchester bomber Salman Abedi's father and younger brother are in detention in Libya.
Abedi, a 22-year-old Briton of Libyan descent who grew up in Manchester, died in Monday's explosion.
Prime Minister Theresa May said "a significant amount of police activity" and several arrests had led to the threat level being lowered. But she urged Britons to remain vigilant and said soldiers would remain at high-profile sites throughout the holiday weekend. The troops will gradually be withdrawn beginning on Monday, she said.
A "severe" threat level means an attack is "highly likely," according to the scale set by Britain's Joint Terrorism Analysis Center. Until it was raised Tuesday, the level had stayed at "severe" since mid-2014.
Assistant Commissioner Mark Rowley, Britain's top counter-terrorism police officer, said authorities have dismantled a "large part" of the network around Abedi, who killed 22 people and wounded dozens by setting off his bomb as people were leaving an Ariana Grande concert Monday in Manchester.
But he said there were still "gaps in our understanding" of the plot, as investigators probed Abedi's potential links to jihadis in Britain, Europe, Libya and the Middle East.
Rowley said the investigation had made "rapid progress" and that police "are getting a greater understanding of the preparation of the bomb."
"There is still much more to do. There will be more arrests and there will be more searches," he said.
Police used an explosive device Saturday to get into a property in Manchester to arrest two men, aged 20 and 22. Investigators have searched 17 properties, including Abedi's home in south Manchester and other houses in nearby districts.
Residents were evacuated from streets in the south Manchester neighborhood of Moss Side in what police called a precaution as the search carried out Saturday. Photos showed an army bomb-disposal unit at the property.
The search was at an apartment in a Manchester high-rise that British media said was rented by Abedi in the months before the attack. Mohammed Hudarey, a friend of the landlord, said that after Abedi moved out about six weeks ago there was a strong smell of chemicals and debris, including metal rods and cut-up fabric, in the apartment.
"We thought he must have been a drug dealer or doing witchcraft," Hudarey told the BBC.
Britain's health service said Saturday that 63 people injured in the bombing remained hospitalized, 20 of them in critical condition. A total of 116 people were treated in hospitals after the bombing.