Britain's home secretary has said the man suspected of killing at least 22 people and wounding dozens at a concert arena in Manchester city "likely" did not act alone.

Amber Rudd told public broadcaster the BBC on Wednesday that suspected suicide bomber Salem Abedi, a 22-year-old Briton, had been known to security services "up to a point".

Her comments came as security was being tightened and police were investigating a "network" over the attack, which was carried out at the Manchester Arena on Monday evening.

Officials are examining Abedi's trips to Libya as they piece together his allegiances and try to foil any new potential threats.

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The suspect apparently detonated a device in the foyer of the concert venue, following a performance by Ariana Grande, an American pop star.

British news media said the youngest victim, Saffie Rose Roussos, was eight years old.

Nearly 3,800 soldiers have been deployed on Britain's streets, taking on guard duties at sites such as Buckingham Palace and Downing Street to free up police to focus on patrols and investigation, Rudd told the BBC.

Police and intelligence agencies detained two more suspects on Wednesday - one in the town of Wigan, west of Manchester - in connection with the attack. The sixth arrest was of a female suspect.

Brother arrested

The latest arrests take the total number in the UK to six, including Abedi's brother Ismail.

"We have made an arrest in Wigan this afternoon in connection with the investigation into the horrific incident at Manchester Arena," a spokeswoman for Greater Manchester Police said.

"When arrested, the man was carrying a package which we are currently assessing."

In a related development, Libyan authorities announced on Wednesday the arrest of Abedi's younger brother, Hashem Abedi, and his father, Ramadan Abedi, in Tripoli.

A counterterrorism force known as Rada detained Hashem Abedi on Tuesday evening, Ahmed Bin Salem, a spokesperson, told Reuters news agency.

The rest of Abedi's family is also thought to be in Libya.

Earlier in the day, police in Britain raided and searched a property elsewhere in Manchester where Ismail Abedi is thought to have lived.

Ian Hopkins, chief constable of Greater Manchester Police, said: "I think it's very clear that this is a network that we are investigating.

"And as I've said, it continues at a pace. There's extensive investigation going on and activity taking place across Greater Manchester, as we speak."

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Authorities have raised the "terrorism" threat to "critical" amid concerns that Abedi may have had accomplices who are planning another attack.

Al Jazeera's Barnaby Phillips, reporting from Manchester, said it was a sophisticated attack - the first successful bomb in the UK for 12 years - and that, as Abedi had only recently returned from Libya, it was unlikely he would have had time to make the bomb himself.

"So, if he was carrying a bomb which had been made by somebody else, then [the questions are] who is that other person, where is he or she at this point in time, and what might they be planning to do," he said.

"All of those are alarming questions for the British authorities and they help explain why the threat level has been raised to critical, why there are now British soldiers on the streets."

Abedi was born in Britain, grew up in Manchester's southern suburbs and attended the local Salford University for a time.

Police on Tuesday raided his house, using a controlled explosion to blast down the door.

Neighbours recalled him as a tall, thin young man who often wore traditional dress and did not talk much.

Theresa May, Britain's prime minister, chaired a meeting on Wednesday of her emergency security cabinet group to talk about intelligence reports on Abedi and concerns that he might have had outside support.

France's interior minister said on Wednesday that Abedi is believed to have travelled to Syria and had "proven" links with the Islamic State of Iraq and the Levant (ISIL).

Gerard Collomb told BFM television that British and French intelligence have information that Abedi had been to Syria.

ISIL responsibility claim

ISIL, also known as ISIS, has claimed responsibility for the attack on its social media channels, saying "one of the caliphate's soldiers placed bombs among the crowds".

However, British officials have not confirmed Abedi's links to ISIL or other armed groups.

In addition to those killed in Monday evening's concert attack, Manchester officials have raised to 119 the number of people who sought medical treatment after the attack, including those who travelled to hospitals on their own.

Jon Rouse of the Greater Manchester Health and Social Care Partnership said on Wednesday that 64 people remain hospitalised,

Officials said 20 of them are being treated for critical injuries.

Many of those still hospitalised had serious wounds that will require "very long term care and support in terms of their recovery", Rouse said.

Officials said all those hospitalised had been identified.

Al Jazeera's Phillips, reporting from Manchester on Wednesday, said there is anger in the city over the attacks.

"What is more of a danger is division and communities turning against each other. I've heard small scale anecdotal stuff to that effect," he said.

"But the overall impression I've had here is of a city doing its absolute best at a time of agony and grief to pull together."

Speaking to Al Jazeera, Andy Burnham, Manchester's mayor, said: "The people of Manchester are bigger than that, they are better than that. They know what the purpose of terrorism is - to divide - and they won't let that happen.

"So there are grounds for hope and the message I will be repeatedly putting out in the next few days is that this man was a terrorist, an extremist. Not a representative of the Muslim community."