“It seems likely — possible — that he wasn’t doing this on his own,” said Britain’s home secretary, Amber Rudd. Speaking to the BBC, she also said the bomb “was more sophisticated than some of the attacks we’ve seen before.”
Chief Constable Ian Hopkins of the Greater Manchester Police said, “There’s an extensive investigation going on, and activity taking place across Greater Manchester.”
Mr. Abedi detonated the bomb Monday night as fans were leaving a pop concert by the American singer Ariana Grande at Manchester Arena. The explosion killed 22 people, including a police officer and an 8-year-old girl. The bomber’s remains were found at the scene, and the Islamic State claimed responsibility for the attack.
At least 64 people were wounded, a third of them critically. Many victims were teenagers and young girls, with parents in tow, who idolized Ms. Grande.
She officially canceled all concerts on her “Dangerous Woman” European tour through June 5 and asked fans to support “all those families affected by this cowardice and senseless act of violence.”
The race to find co-conspirators and the place where the bomb had been made appeared to be the main reasons behind the British government’s decision on Tuesday to raise the terrorist threat warning to critical, its highest level since 2007, over fears that more bombs could be detonated in crowded places.
The police arrested five men and one woman in the Manchester area — bringing the total number of people in British custody to seven, including Mr. Abedi’s older brother.
In Libya, Mr. Abedi’s father was arrested by a militia, the Special Deterrence Forces, which said it also had detained Mr. Abedi’s younger brother, Hashem Abedi, 20.
In a Facebook post, the militia said that Hashem Abedi had been a member of the Islamic State, was tied to the Manchester plot and was en route to withdrawing 4,500 Libyan dinars (about $560 on the black market) sent by the bomber when he was arrested on Tuesday night by the militia.
The militia said that Hashem Abedi had traveled from Britain to Libya on April 16, that he had been planning an attack in Tripoli and that he had been in daily contact by phone with his older brother.
The militia’s claims about the younger brother could not be immediately verified. The militia is affiliated with the United Nations-backed Government of National Accord, one of three administrations vying for control of Libya, but it has been accused by human rights groups of abusing prisoners.
Besides the younger brother, the authorities were pursuing many leads. The BBC reported that officials believed Salman Abedi may have been a “mule,” carrying a bomb made by someone else. The officials also said they were looking into Mr. Abedi’s relationship with Raphael Hostey, a British recruiter for the Islamic State believed to have been killed in a drone strike in Syria last year.
In Washington, a senior American official said Mr. Abedi had links to a radical preacher in Libya identified as Abdul Ghwela, whose son had joined the Islamic State in Libya and had died fighting in Benghazi. The official, who spoke on the condition of anonymity to disclose intelligence information, said Mr. Abedi had not left Libya until May 17.
In addition, officials were looking into reports that people who knew Mr. Abedi — including an imam at his mosque — had contacted the authorities as early as 2015 with concerns that he may have been recruited by extremists.
The heightened warning of additional, possibly imminent attacks was visible nationwide. The government suspended public tours of Parliament and canceled the guard-changing ceremony at Buckingham Palace, a tourist favorite. Soldiers patrolled locations including Downing Street, where the home and office of the prime minister are, and foreign embassies.
Manchester, a city of half a million and the hub of Britain’s second-largest metropolitan region, is home to a sizable community of people of Libyan descent. Many fled the regime of the longtime dictator Col. Muammar el-Qaddafi in the 1980s. The violent overthrow of Colonel Qaddafi in 2011 during the tumult of the Arab Spring created a power vacuum, and the Islamic State and other extremist groups have since gained support.
Many Libyan expatriates are clustered in Manchester, creating one of the largest Libyan communities outside Libya, according to Nazir Afzal, who until 2015 was the chief prosecutor for northwest England, based in the city.
Among them was the Abedi family, which moved to Britain in 1993. Salman Abedi was born there a year later.
Reached by phone in Libya on Wednesday, Ramadan Abedi, his father, expressed shock and denied that his son was the bomber.
“I don’t believe that it was him,” he said. “His ideas and his ideology were not like that.”
Mr. Abedi confirmed that his son had been distressed by the murder of a friend, Abdul Wahab Hafidah, in May 2016 at the hands of suspected gang members. But he said it did not drive him toward radicalism.
The father’s account was contradicted by several people who knew the family, including one quoted by the BBC who said Salman Abedi had expressed approval of suicide bombers a few years ago, leading neighbors to call an antiterrorism hotline.
The French interior minister, Gérard Collomb, said on Wednesday that Mr. Abedi had “most likely” gone to Syria and that he had “proven” links to the Islamic State.
Mr. Abedi’s parents, who moved back to Libya after Colonel Qaddafi’s downfall, had become worried about their son’s radicalization, and they had even seized his British passport, according to a friend in Manchester who spoke on the condition of anonymity because he feared for his safety.
Mr. Abedi had told his parents that he wanted to visit the holy city of Mecca, so they returned his passport. But instead he returned home, the friend said.
The father denied that account. “He was a man and I trust the man that he was,” he said. A short while later, the father was arrested in Tripoli, according to the same Islamist militia that announced the younger brother’s arrest.
A number of Libyans from Manchester have waged jihad abroad, according to Raffaello Pantucci, a terrorism expert at the Royal United Services Institute in London. The Qaeda-linked Libyan Islamic Fighting Group had a contingent in Manchester, Mr. Pantucci said. And in 2010 and 2011, as the anti-Qaddafi uprising in Libya intensified, a number of Libyan-Britons left Manchester for Libya as foreign fighters, he said. More recently, he said, a cluster left for Syria.
In Fallowfield, a neighborhood south of the Manchester city center, residents recalled Mr. Abedi as quiet, respectful and passionate about soccer, often wearing a T-shirt with a Manchester United emblem.
Officials at the Manchester Islamic Center, also known as Didsbury Mosque, where the Abedi family worshiped, have condemned the attack, but declined to talk about the family.
“The horrific atrocity that occurred in Manchester on Monday night has shocked us all,” a mosque trustee, Fawzi Haffar, told reporters.
In 2015, according to a neighbor who spoke on the condition of anonymity over concerns about safety, an imam at the mosque, Mohammed Saeed, delivered a sermon condemning terrorism for political causes. The sermon prompted a heated discussion among congregants and some, including Mr. Abedi, objected to it.
“He was angry,” the neighbor said. “He scared some people.”