British authorities are trying to determine who was part of the network that might have helped the bomber, Salman Abedi, carry out the attack at the Manchester Arena.
■ The British man officials say carried out the Manchester bombing, Salman Abedi, was most likely part of a “network” that might have helped carry out the attack on Monday that killed 22 people, and could be planning further assaults.
■ His younger brother, Hashem Abedi, 20, was arrested in Libya by a powerful Islamist militia that has accused the younger Mr. Abedi of being a member of the Islamic State.
■ Salman Abedi recently traveled to Libya, his family said, and he may have visited Syria according to security officials. Mr. Abedi’s father in Libya said he did not believe his son was responsible for the Manchester attack.
■ Britain mobilized armed forces to bolster security at vital locations around the country after Prime Minister Theresa May on Tuesday put the country on the highest level of alert, “critical, ” meaning that another attack “may be imminent.”
View photographs from the investigation and aftermath of the attack here .
Investigators have been focused on determining who might have helped the bomber, Mr. Abedi, plan and execute the attack at the Manchester Arena after an Ariana Grande concert.
“It seems likely — possible — that he wasn’ t doing this on his own, ” Britain’s home secretary, Amber Rudd, said on Wednesday.
Chief Constable Ian Hopkins of the Greater Manchester Police said at a news conference on Wednesday, “This is a network that we are investigating.”
He did not specify whom the police were looking for, or if investigators were searching for a bomb maker.
“There’s an extensive investigation going on, and activity taking place across Greater Manchester as we speak, ” he said.
The BBC, citing unidentified intelligence sources, reported on Wednesday that officials believed Mr. Abedi had been a “mule, ” carrying a bomb made by someone else.
The police said they arrested three men on Wednesday, but what role, if any, they may have played in the attack is unclear.
Mr. Abedi, born in Manchester in 1994, recently visited his parents who had moved back to Libya after decades living in Britain. His father, speaking by phone from Tripoli, the Libyan capital, said he believed his son was on his way to make a religious pilgrimage at the time of the attack.
“I don’ t believe that it was him, ” said the father, Ramadan Abedi, known as Abu Ismail. “His ideas and his ideology were not like that. He was born and raised in Britain. He’s a British citizen and he does not hold such ideologies.”
A powerful Islamist militia in Tripoli said on Wednesday that it had arrested Mr. Abedi’s 20-year-old brother, Hashem, and that it was holding him at its base in the Libyan capital.
Hashem Abedi was arrested Tuesday night at the Abedi family’s home in Tripoli as news of Salman’s involvement in the Manchester attack was widely reported, according to Ahmed Omran, a spokesman for the militia, known as the Special Deterrence Force.
“We have been following him for at least a month and a half now, ” Mr. Omran said, adding that the group suspected Hashem was a member of the Islamic State.
The Special Deterrence Force, also known as Rada, is one of the most powerful militant groups on the streets of the often lawless Libyan capital. Its leaders are staunch Islamists and it operates a detention facility where many people suspected of being Islamic State fighters have been held.
The group is currently affiliated with the United Nations-supported Government of National Accord, one of three administrations vying for control of Libya.
Friends and neighbors of Salman Abedi have said he expressed extremist views, and security officials have said he traveled to Syria, the stronghold of the Islamic State.
In 2015, according to a neighbor of the Abedi family who also spoke on the condition of anonymity because of concerns about safety, the imam at the mosque attended by the family delivered a sermon condemning terrorism and murder carried out in the name of a political cause. The sermon by the imam, Mohammed Saeed, prompted a heated discussion among congregants, some of whom signed a petition taking issue with the sermon, according to the neighbor.
“He was angry, ” the neighbor said of Mr. Abedi. “He scared some people.”
A friend of Salman Abedi, who spoke on the condition of anonymity because he feared for his safety, said that Mr. Abedi, along with other young men of Libyan descent in Manchester, had been deeply angered by the gang killing last year of a friend, Abdul Wahab Hafidah.
It was around this time, according to people who knew the family, that Mr. Abedi had traveled to Libya to see his parents. He also traveled to Syria, according to the French interior minister, Gérard Collomb.
Mr. Collomb said that Mr. Abedi had “proven” ties to the Islamic State, but did not give details. It was not clear whether Mr. Abedi’s visits to Libya and Syria were part of the same trip.
During Mr. Abedi’s visit to Libya, his parents had become worried about his radicalization, and had even seized his British passport, according to the friend in Manchester.
Mr. Abedi’s father denied this. He said in a telephone interview on Wednesday that his son was not an extremist, and that he had his passport because he was traveling back to Manchester before making his way to Saudi Arabia for a religious pilgrimage.
Officials from the mosque attended by the Abedi family said on Wednesday that anyone with information about Mr. Abedi should cooperate with the police investigation.
Speaking outside the Manchester Islamic Center, also known as the Didsbury Mosque, one of the mosque’s trustees, Fawzi Haffar, said: “We encourage anyone, and I repeat, we encourage anyone who may have information about the individual involved to contact the police without delay.”
The military, under the code name Operation Temperer, has been deployed to vital locations around Britain to assist police officers in protecting against another terrorist attack.
“This frees up armed police officers to then give the police service the capacity to then deploy them to places like Manchester, ” Chief Constable Hopkins said, adding: “There are no military personnel patrolling the streets of Greater Manchester nor are there plans for that to happen at this time.”
The British police identified Mr. Abedi on Tuesday after several United States news organizations, including CBS News and USA Today, citing American intelligence sources, had already done so.
Ms. Rudd, Britain’s home secretary, made clear on Wednesday the government’s dismay at the leaks.
“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, ” she told BBC Radio 4. “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.
