MANCHESTER, England — Seven additional people were taken into custody in Britain and Libya on Wednesday in connection with the suicide bombing that killed 22 people at a pop concert in Manchester, as officials examined the attacker’s possible ties to jihadis in the Middle East.

As Britain deployed hundreds of soldiers to the streets and the nation remained under “critical” alert for another possible terrorist attack, investigators searched several locations around England for evidence that might tie the English-born bomber, Salman Abedi, 22, to a larger terrorist network.

Islamic State has claimed responsibility for the attack, and new reports that Abedi had traveled to Libya and Syria, home to the militant group’s self-declared caliphate, probably raise serious red flags for investigators seeking to confirm whether the group managed to orchestrate a devastating attack inside Britain.

The nine people now in custody include two of Abedi’s brothers and his father, a Libyan security official who had fled to Britain as a dissident of the Moammar Gadhafi government in the 1990s before returning in 2011. Salman and at least one of his brothers remained in England.

Before his arrest in Tripoli on Wednesday, the father, Ramadan Abedi, 51, had protested in an interview with The Associated Press that his son was innocent and had been preparing to go on a pilgrimage soon to Saudi Arabia.

“We don’t believe in killing innocents. This is not us,” said Abedi, who is an administrative manager of the Tripoli Central Security Force, which is aligned with Libya’s U.N.-backed government. “We aren’t the ones who blow up ourselves among innocents. We go to mosques. We recite Koran, but not that.”

Abedi said his son had sounded normal the last time he spoke to him. “There was nothing worrying at all until two days ago (when) I heard the news that they suspect he was the bomber.”

Salman Abedi’s younger brother, Hisham, also was arrested in Libya on Tuesday, according to a statement from the Libyan Special Deterrence Forces.

Hisham Abedi told the authorities in Libya that he had been in Britain during the preparation for the bombing, and “it was clear that he was aware of all the details about the attack,” the statement said.

Another of the bomber’s brothers, Ismail, was arrested in the Manchester area shortly after the Monday evening attack, but officials have not said why.

Security services continued to work around the clock to determine what motivated Abedi, who was born in Manchester, to blow himself up just outside the exits of the 21,000-seat Manchester Arena moments after American singer Ariana Grande had finished her packed concert. Grande, whose fans include legions of teenage girls, has since canceled her shows through June 5, which were to be held in London and Switzerland.

British Prime Minister Theresa May has raised the country’s threat level from “severe” to “critical,” indicating another attack might be imminent, marking the first time in 10 years that the nation had reached the highest possible threat level.

Armed soldiers were visible at key locations around the country Wednesday, including outside Downing Street, the houses of Parliament and Buckingham Palace.

Authorities said as many as a thousand soldiers were mobilizing to assist armed police and help protect key locations such as airports, transport hubs and nuclear plants.

But political leaders already were preparing to resume campaigning Thursday and Friday for parliamentary elections set for June 8.

“The spirit of Manchester — and the spirit of Britain — is far mightier than the sick plots of depraved terrorists,” May said after meeting with her security chiefs.

The latest arrests include five men and one woman in the Manchester region, plus one man in Nuneaton, a town outside Birmingham about halfway between London and Manchester. Officials have not released names or given details about the accusations against the suspects.

Given the nature of the attack and the bomb used — filled with nails and bolts designed to cause maximum carnage — Home Secretary Amber Rudd said Wednesday that it was likely the 22-year-old had not acted alone.

“It was more sophisticated than some of the attacks we’ve seen before,” she said.

News that Abedi had traveled to Syria came from France’s interior minister, Gerard Collomb, who told BFMTV that the bomber had proven links to Islamic State.

“Today we only know what British investigators have told us — someone of British nationality, of Libyan origin, who suddenly after a trip to Libya, then probably to Syria, becomes radicalized and decides to carry out this attack,” he said.

Much of the information about the Manchester investigation has come from law enforcement sources outside Britain who have been briefed by their British counterparts. The leaks have irritated officials in Britain, where such investigative information is not shared with the media as commonly as it is in the U.S.

“The British police have been very clear that they want to control the flow of information in order to protect operational integrity, the element of surprise,” Rudd said. “So it is irritating if it gets released from other sources, and I have been very clear with our friends that should not happen again.”

Rudd said that Abedi had been known to intelligence services “up to a point” and that his recent trips to Libya, where his parents live, are being investigated.

Hamid El-Sayed, an employee at the University of Manchester, told the BBC that Abedi’s family had tried but failed to keep him on the “right path.”

“Eventually he was doing very bad at his university, at his education, and he didn’t complete, and they tried to take him back to Libya several times,” El-Sayed said. “He had difficulties adjusting to European lifestyle.”

Fawzi Haffar, a trustee of the Manchester Islamic Center and Didsbury Mosque, denied news reports that Abedi had worked there in the past.

“This is not true. I assure everyone. Listeners, viewers in the U.K., around the U.K., this bomber has never worked in this center,” he said.

Haffar did not directly answer questions about whether Abedi and his father prayed at the mosque, which also has been alleged, although a prayer leader at the mosque told the Los Angeles Times that he had seen the bomber at the mosque previously.

“This center has been part of our fine and great city and the Didsbury community,” Haffar said. “This has been going on since the 1960s. The doors of the center are open to all.”

He pleaded with anyone with any information about Abedi to contact the police without delay and expressed concern about reports he was receiving of a rise in anti-Muslim acts in recent days, ranging from verbal abuse to criminal damage of mosques.

“These incidents have to be reported as hate crimes,” he said.

As the identities of the victims became public, the indiscriminate toll of the bombing became clearer.

Among those killed: an 8-year-old girl, an aunt who relatives said shielded her niece from the blast, parents waiting to pick up young concertgoers; and a Polish couple from York, a city in the north of England.

An off-duty female police officer also was killed.

And Olivia Campbell, 15, whose mother, Charlotte, had put out a desperate and heartbreaking plea for information about her daughter’s whereabouts Monday, was finally confirmed among the dead.

“Go sing with the angels and keep smiling. Mummy loves you so much,” Charlotte Campbell wrote on Facebook.

In a week of memorials, another vigil was being held Wednesday evening, this one in the Bury district of Manchester. It was for one of the victims who lived in the area, John Atkinson, 26 — and for Olivia.

Police said they are now confident they know the identity of all 22 victims in Manchester, but autopsies will take four to five days because of the number of cases.

Sixty-four people are being treated in hospitals across Manchester, 20 of whom are dealing with life-changing injuries, including “major wounds, damage to organs, embedded objects,” said Jon Rouse, chief officer of the Greater Manchester Health and Social Care Partnership.

Campaigning for the June elections had been suspended, but the leader of the UK Independence Party said Wednesday it would release a party manifesto Thursday, and the Labor Party is likely to return to the campaign trail Friday but in a low-key fashion.

“The best response we can make is to ensure that the democratic process continues,” said Paul Nuttall, leader of the UK Independence Party.

There was no indication that any of the other main parties planned to resume campaigning anytime soon.

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(Special correspondents Boyle and Kelly reported from London and Manchester, respectively, and Times staff writer Pearce from Los Angeles. Staff writer Melissa Etehad in Los Angeles contributed to this report.)