Britain is under a sweeping security buildup with close to 4,000 soldiers expected to start patrolling city streets as fears of another terrorist attack increase.

Nearly 1,000 armed soldiers began guarding landmarks across the country on Wednesday, including the Houses of Parliament, 10 Downing St. and Buckingham Palace. Officials also closed Parliament to the public, cancelled the changing of the guard at Buckingham Palace and shut public galleries at the Old Bailey court house. Home Secretary Amber Rudd said a total of 3,800 soldiers will be deployed over the next few days in addition to thousands of extra police.

The country is under unprecedented pressure, with a shaken public, an election campaign on hold and rising tensions over race and religion. There are also growing concerns that efforts to combat radical extremists have been dealt a major setback. After years of insisting that police intelligence gathering had been improved across the country, questions are being asked about how a 22-year-old university dropout could carry out a bombing powerful enough to kill at least 22 people and injure 59 others at a packed concert arena.

On Wednesday, British police broadened their investigation into Monday night’s bombing at Manchester Arena, arresting six people associated with the suicide bomber, Salman Abedi. The sprawling probe also extended to Libya. Police say the bomb was designed for massive destruction and was packed with nuts and bolts that sprayed across the arena’s foyer just after a performance by pop star Ariana Grande.

At least 16 victims have been named, including an eight-year-old girl, a couple from Poland, an off-duty police officer and a 51-year-old school receptionist. One victim, 32-year-old Kelly Brewster, died while shielding her two nieces from the explosion, her family said. The young girls remain in hospital with broken bones and shrapnel injuries. Police said they have identified all of the dead, but won’t formally release the names until forensic post-mortems have been completed.

Manchester police raided a house in the centre of the city Wednesday. So far six men and one woman are in British custody including Mr. Abedi’s brother, Ismail, who is 23. His younger brother, Hashem, 20, has also been arrested in Libya, where he lived with his parents, on suspicion of links with the Islamic State.

“It’s very surreal,” said Adam Prince, 38, who arrived back to his flat in the heart of Manchester’s gay district on Wednesday afternoon only to discover that police had raided an apartment on his floor in connection with the probe. Mr. Prince said the apartment is regularly rented out on Airbnb but he had not seen the latest occupant. The extra police presence around the building and in the heart of the city has shaken him. “There are so many people that love this city so to see people try to destroy the city in some way is just heartbreaking,” he said.

Police have released few details about Mr. Abedi, but reports say he was born in Britain and that his parents came to the country as refugees in the 1990s from Libya. He attended local schools and briefly studied business management at nearby Salford University. His parents returned to Libya six years ago, but Mr. Abedi and his older brother stayed behind. However, reports say Mr. Abedi went to Libya several times and recently returned from a three-week-long trip. Much of the North African country has become virtually lawless since the fall of long-serving ruler Colonel Moammar Gadhafi in October, 2011, and parts are now controlled by the Islamic State.

“We don’t believe in killing innocents. This is not us,” his father, Ramadan Abedi, told reporters in Libya shortly before he was held for questioning by Libyan police. “My son was as religious as any child who opens his eyes in a religious family. As we were discussing news of similar attacks earlier, he was always against those attacks, saying there’s no religious justification for them.”

The Abedi family attended the Didsbury Mosque in South Manchester and there are reports some people in the neighbourhood had concerns about him and contacted police several years ago. Manchester police have declined comment.

On Wednesday, many of those attending daily prayers at the mosque were struggling to come to grips with bombing. “This act of cowardice has no place in our religion. … We encourage anyone with information to contact police,” said Fawzi Haffar, who spoke on behalf of the mosque.

Hamila Khan expressed outrage at the attack as she arrived for prayers. But she also aimed her anger at people who condemn all Muslims for the actions of a small number. “Islam is a religion of peace. You’re not allowed to take the life of another human being because it’s hellfire for you,” she said. “This is my country. This is my community. I’ve done my best. I want the best for Manchester and for Britain.”

When asked if she knew Mr. Abedi, Ms. Khan shot back: “I don’t know this gentleman. How many people do you know in your church? How many do you speak to?”

Tension briefly heightened when a man stopped on the sidewalk to yell at Mohammed Fadeil, who was entering the mosque. “You need to sort this thing out in your community, and put a stop to it,” said the man, named Ian MacIntosh. “It could have been my 19-year-old daughter going to that concert.”

Afterward, Mr. Fadeil appeared shaken by the encounter. “This is what the terrorists want,” he said. “They want to divide us. I’ll reach out to him. We’re Muslims, we’re here to fight this together, join us.”

One 30-year-old man standing outside the mosque offered an explanation for the bombing, saying that while he condemned all killing he could understand why people such as Mr. Abedi become suicide bombers. In Iraq, Syria and Afghanistan “Muslims are being killed left, right and centre and nobody cares,” said the man who declined to give his name but said he attended the mosque.

That kind of response infuriated Mohammed Shafiq, who runs the Ramadhan Foundation, a Manchester-based organization that promotes peaceful co-existence. “I get some of that, to be honest, on Facebook,” Mr. Shafiq said. “But at the end of the day, it’s children that have been brutally murdered for no other reason than they had gone to a music concert. And nobody should justify that.”

He believes Muslim leaders need to do more to challenge the ideology of the Islamic State and others on a theological basis. And he said there has been too much disconnection in mosques between young people and imams.

“We’ve got to take on the theological battle which is that Islamic State and al-Qaeda justify their violent methods by using the verses from the Koran, using the sayings of the prophet Mohammed. … We’ve got to take them on because we believe their distortion is leading to a wrong image of Islam.”

The South Manchester area has become one of the country’s hotbeds for radical extremists. It was home to Raphael Hostey, who became a leading Islamic State recruiter before he was killed in a drone strike last year. Earlier this year, another man from the area, Ronald Fiddler, blew himself up in a suicide attack in Mosul. Police have long struggled with the powerful online propaganda employed by the Islamic State, which Mr. Shafiq said brainwashes vulnerable young people.

“We’ve got to face this extremism head on and I think there’s a responsibility on the part of us in the Muslim community,” he said. “Have we done enough? I don’t think we have.”

MANCHESTER — The Globe and Mail
Published Wednesday, May 24, 2017 4:11PM EDT
Last updated Wednesday, May 24, 2017 10:03PM EDT