Manchester suicide bomber Salman Abedi likely received some ISIS training by traveling to Syria in the months before the attack, according to information gathered in the preliminary investigation, a US official told CNN on Thursday.
The US believes ISIS, through that training, set the stage for the 22-year-old to carry out his deadly attack at the Manchester Arena on Monday night. Other members of Abedi's family are believed to have been radicalized as well, the official said.
The development comes as police raids continued, "specially trained firearms officers" were assigned patrol duty on trains for the first time, and Britain temporarily halted intelligence sharing on the attack with the US after American officials leaked information to the media.
Late Thursday, however, the National Police Chiefs' Council said in a statement that it had resumed intelligence sharing.
The council's assistant commissioner, Mark Rowley, said "fresh assurances" ensure that "we are now working closely with our key partners around the world including all those in the 'Five Eyes' intelligence alliance." The US, UK, Canada, Australia and New Zealand share intelligence in one of the world's tightest multilateral arrangements.
US officials released the bomber's name and other details of the investigation into the deadly attack, irking British investigators scrambling to keep details of the probe confidential.
Prime Minister Theresa May said she would raise the leaks with Trump when the pair met at a NATO summit in Brussels, Belgium.
Arriving at the summit, she told reporters the "special relationship" with the US on intelligence sharing is based on trust.
Trump, in turn, vowed to investigate, according to a White House statement to reporters, saying "leaks of sensitive information pose a grave threat to our national security."
"The alleged leaks coming out of government agencies are deeply troubling. These leaks have been going on for a long time and my administration will get to the bottom of this," he said.
"I am asking the Department of Justice and other relevant agencies to launch a complete review of this matter, and if appropriate, the culprit should be prosecuted to the fullest extent of the law. There is no relationship we cherish more than the special relationship between the United States and the United Kingdom."
In Washington, US Attorney General Jeff Sessions said in a statement that he shared Trump's "deep concern" about the leaks, which he discussed with UK Home Secretary Amber Rudd.
"These leaks cannot be tolerated and we will make every effort to put an end to it," Session said. "We have already initiated appropriate steps to address these rampant leaks that undermine our national security."
Addressing leaders ahead of the NATO summit, Trump paid tribute to the victims of the attack, saying it was "a barbaric and vicious attack upon our civilization."
He dismissed terrorists as "losers" and urged nations to unite to ensure terrorism is "stopped in its tracks."
US Secretary of State Rex Tillerson will make his first official visit to the UK on Friday to meet with British Foreign Secretary Boris Johnson "in an expression of UK-US solidarity following the terrorist attack in Manchester earlier this week," a Foreign Office statement said. The two men will write condolence messages for the victims.
Armed officers will patrol trains
In the aftermath of the attack and the raising of Britain's threat level to "critical" -- the highest level -- British Transport Police announced that "specially trained firearms officers will be be patrolling on board train services for the first time."
"Armed officers have been regularly patrolling on the London Underground since December, but this will be the first time they will patrol on board train services elsewhere in the country," a BTP statement said.
BTP Chief Constable Paul Crowther said there's no specific threat, but it's part of the national mobilization plan "to deal with the current and ongoing threat.
"It is important to note that we do not have any specific intelligence in relation to train services but are taking this action to ensure we can protect and reassure the public," Crowther said in the BTP statement.
Mayor: We can't afford the risk
Regarding information sharing with the United States, Greater Manchester Mayor Andy Burnham said that only intelligence sharing relating to the Manchester probe had been suspended, adding that "we quite frankly can't afford to risk it any more."
"It has been compromised by the leaks and we can't afford any more, so we've taken a temporary decision to say let's clear it out and draw a line, let's not have a row that lingers on. Nobody wants that. But equally we have to make our opinions clear so we can get this relationship back to where we all want it to be," Burnham told CNN.
US sources were the first to reveal the identity of the bomber, leading to concern that police efforts to hunt down his associates could be impacted.
On Wednesday, The New York Times posted photos that show what it said could be the detonator, a battery, nuts and screws for shrapnel, and fragments of a backpack used in the attack. The paper, without specifying the source, said British authorities provided access to photos of materials found at the scene.
Greater Manchester Police Chief Constable Ian Hopkins said the publication of leaked intelligence by the New York Times had "caused much distress for families that are already suffering terribly with their loss."
Former White House counterterrorism chief Richard Clarke told CNN that he didn't "blame the British one bit for turning off the flow of information."
"You do run a risk of some piece of information that could have stopped another attack won't get through. So because of this climate of lack of trust, there are risks to lives, frankly -- to citizens who might be the victims of the next attack," he added.
Monday's attack, which came as people were leaving an Ariana Grande concert, took the lives of at least 22 people, including several children. Dozens were wounded in the incident.
Investigators are focused on tracking down associates of Abedi, a British-born national of Libyan descent, saying it is clear they are investigating a network.
Hopkins told reporters eight men had been arrested at addresses in Manchester, Wigan and Nuneaton. These arrests are "significant," he said, with searches having "revealed items that we believe are very important to the investigation."
On Thursday, a large cordon was set up for a time around a location in Wigan, where an arrest was made a day earlier, Greater Manchester Police said in a statement.
A raid and search have been ongoing due to the discovery of "potentially suspicious items" and evacuations took place as a "matter of precaution."
A woman arrested in a raid in Blackley was released late Wednesday without charge, police said.
Other details emerged about Abedi, including that he had been known to intelligence services, Rudd told the BBC. He had been in Libya for three weeks and returned days before his attack, US military officials told CNN.
Police in Germany confirmed that Abedi had traveled through Dusseldorf Airport on his way to Manchester a few days before the attack. The suspect was in transit only in the security area, Dusseldorf police said.
A Turkish official said Abedi had transited through Istanbul's Ataturk airport "recently" and did not enter Turkey. The official, who did not want to be named due to government protocol, said he could not confirm where Abedi arrived from or was traveling to. He did say, however, that it could not have been Syria because there are no flights to or from Syria from Istanbul.
The official said Turkey had not received any information from foreign intelligence services about Abedi, so he did not raise any red flags.
In Libya, Abedi's brother -- identified as 20-year-old Hashim Ramadan Abu Qassem al-Abedi -- was arrested Tuesday night on suspicion of links to ISIS, according to a statement from a Tripoli militia known as the Special Deterrence Force. The militia, aligned with the Libyan government, said that the younger Abedi was allegedly planning a terror attack in Libya when arrested. CNN has not been able to independently verify the details from the militia.
According to a family friend who asked not to be identified, the boys' father had taken his sons to Libya in mid-April and confiscated their passports so they couldn't return to the UK where they'd been in trouble with gangs. Abedi got his passport back after telling his father he was going on a pilgrimage to Saudi Arabia, but returned to England instead.
UK holds moment of silence
Identification of the 22 victims continues. As of Thursday afternoon, 18 people who died in the blast had been named.
The family of 14-year-old Eilidh MacLeod released a statement through the police that spoke of their "devastation."
The grandfather of another teenage victim, Sorrell Leczkowski, said he was "absolutely heartbroken" to confirm that she had died. "Sorrell was only 14, but she was our rock, she kept us all grounded. She was such a clever, talented, creative girl, there was nothing she couldn't do," said Michael Healy.
Queen Elizabeth II visited the Manchester Royal Infirmary and Children's Hospital, where some of the wounded are being treated. Speaking to the father of an injured girl by her hospital bed, the queen said the attack was "a very wicked thing."
The family of Laura MacIntyre, in critical condition after the blast, released a statement that posted to the Manchester police Twitter account. MacIntyre's friend, Eilidh MacLeod, 14, died during the attack.
"Our daughter Laura is a funny and witty young girl who excels at everything she does," the statement said. "Laura and her friend Eilidh were so looking forward to the concert, but that night has now ended in tragedy where Eilidh has lost her life and Laura is in critical condition. Our hearts and minds have been with Eilidh's family since we heard their news."
Manchester hospitals are still treating 75 people in the aftermath of Monday's bombing attack, including 23 in critical care, according to a statement from NHS England.
Professor Bob Pearson, Medical Director of Central Manchester Hospitals Trust said 32 patients remained at the Manchester Royal Infirmary, including five children and five adults in critical care.
"Many of the injuries were horrific and potentially life-changing for the patients involved," he said.
A moment of silence was held Thursday morning as the United Kingdom continued to come to terms with its worst terror attack since the 2005 London bombings.