British Home Secretary Amber Rudd said there was still a chance that some attack accomplices were still at liberty.

"Potentially," she said, when asked if some of the network members might be at large.

Rudd made the comments as British police continued their search for members of a terror cell behind a bombing attack in Manchester on May 22 that left 22 people dead.

Read more: In Manchester, a show of determination, anger, fear and love

"The operation is still at full tilt," she said. Police say that 1,000 people are working on the investigation, trying to track down bomber Salman Abedi's accomplices and piece together his movements in the days before the attack.

"Until the operation is complete, we can't be entirely sure that it is closed," Rudd said.

Abedi, a 22-year-old Briton of Libyan descent, detonated a bomb-laden backpack at the end of an Ariana Grande concert last Monday. Police on Sunday released new images of him in the hours leading up to the attack in an effort to garner potentially crucial information about his movements on that day. Police have previously said they fear Abedi may have passed on a second explosive device. 

British police are currently holding 13 suspects, including Abedi's older brother Ismail. The latest arrest was made on Sunday in the Gorton area of Manchester. In a statement, Manchester Police said they had detained a 19-year-old on "suspicion of offences contrary to the terrorism act." Another brother and Abedi's father are being held in Libya.

Prime Minister Theresa May said developments in the investigation prompted intelligence experts to lower the threat level in the country from "critical" to "severe" on Saturday, claiming that a critical part of the network had been dismantled.

Still, security remains high at large-scale public events, including the Great Manchester Run where some 40,000 runners took to the city streets.

British officials confirmed that Abedi recently returned from Libya on May 18, and that police were seeking more information about his movements upon his return.

Abedi was known to British security officials before the bombing, the government said, but Rudd declined to comment on exactly what had been known about him.

Hundreds of plots

Media reports say that people who knew Abedi had raised concerns about him and his views as far back as 2012.

"The intelligence services are still collecting information about him but I wouldn't rush to conclusions, as you seem to be, that they have somehow missed something," Rudd said.

Read more: Manchester bomber: A life on the move

When asked how many potential militants the government was concerned about, Rudd said Britain's security services were tracking 500 different potential plots, involving 3,000 people as a "top list", with a further 20,000 beneath that.

"That is all different layers, different tiers, it might be just a question mark about one of them or something serious with that top list," she said.

The British government has previously complained that technology companies are not doing enough to clampdown on would-be terrorists who use IT networks both to promote extremist ideology and for communication between militant suspects via encrypted messages.

Rudd said Britain was making good progress with internet firms but that more needed to be done. Technology companies such as WhatsApp say they cannot break end-to-end encryption.

"I believe we can get them to be more successful in working with us to find a way of getting some of that information," she said. "The area that I am most concerned about is the internet companies who are continuing to publish the hate publications, the hate material that is contributing to radicalizing people in this country."

bik/rc (AP, Reuters)