British police say they are again sharing intelligence about the Manchester bombing with U.S. police and intelligence agencies. They had briefly halted the sharing of information with the U.S. after details were leaked to U.S. media.

British Prime Minister Theresa May said on Thursday that eight people are in custody — following the arrests of two more people earlier in the day — as the investigation continues into the deadly attack at an arena where American singer Ariana Grande had just finished her performance.

But the public "should remain vigilant," May said during a brief televised address after a meeting of the government's COBRA crisis committee.

Threat level still critical

May said the national threat level remains critical — meaning another attack may be imminent.

The threat level was raised to its highest level after suicide bomber Salman Abedi killed 22 people at the concert Monday night at Manchester Arena. Police say he was part of a network and they are racing to track down his links.

British police and other security forces have complained bitterly about leaks of sensitive material related to the investigation by U.S. officials.

Officials are particularly angry that photos detailing evidence about the bomb were published in the New York Times. The photos include the remains of a 12-volt battery — thought to be the bomb's power source, according to the article — and a bloodied cylinder that was reportedly found in the bomber's left hand. The item might be the bomb's trigger, or nothing more than the remains of an e-cigarette, according to one explosives expert who spoke to the Times.

Manchester police Chief Ian Hopkins said the story and photographs were "distressing" to victims' families.

This item, seen in a photo published by the New York Times, was reportedly found in the bomber's hand and could be the remains of the trigger that set off the explosion. (New York Times)

Reuters spoke to officials at six U.S. intelligence and defence agencies, all speaking on the condition of anonymity, who said they don't expect British government concerns about U.S. officials' leaks on the Manchester suicide bombing to affect the two countries' close intelligence partnership.

"The value of the intelligence that we and our allies share on terrorism and other issues is far too important to limit because of some newspaper pictures and the premature publication of the bomber's name," said one of the officials, who is familiar with long-standing agreements that include Britain, the United States and other countries.

As well, two of the U.S. officials, from different agencies, told Reuters the photos of remnants of the Manchester bomb, and other evidence that U.S. officials allegedly leaked were of what one of them called "limited intelligence and law enforcement value."

U.S. officials also leaked Abedi's name to the media earlier this week, before it had been released in the U.K. British Home Secretary Amber Rudd complained that such leaks could cost police "the element of surprise" in their bid to prevent future attacks.

British officials are upset that bomber Salman Abedi's name was leaked by U.S. officials and published while U.K. police were withholding the name for reasons of operational security. (Associated Press)

Britain and the U.S. have traditionally shared intelligence at the highest levels and — together with Canada, Australia and New Zealand — are members of the "Five Eyes" intelligence-sharing alliance.

British liaison officers work at the CIA, the electronic eavesdropping National Security Agency, the Office of the Director of National Intelligence and elsewhere.

U.S. military and civilian counterparts are stationed in Britain including a contingent at the Cheltenham headquarters of the NSA's British counterpart, the Government Communications Headquarters, or GCHQ.

Intelligence officers from both countries also work closely in third countries, including war zones such as Iraq and Afghanistan.

Trump said his administration will "get to the bottom" of the leaks and pursue possible prosecution.

"There is no relationship we cherish more than the special relationship between the United States and the United Kingdom," he said in a statement.

The U.S. president has for months been troubled by leaks — often relating to the investigation of Russia's alleged meddling in the election that brought him to power — though he personally came under fire when it emerged he had revealed highly classified information, that had been shared by Israel, to Russian officials during a recent meeting at the White House.

Asked whether he had any concerns about intelligence sharing within NATO, Prime Minister Justin Trudeau said Canada will continue to "collaborate" as usual with its allies.

The New York Times defended publishing the material, saying in a statement that "the images and information presented were neither graphic nor disrespectful of victims."

It said its coverage of the attack "has been both comprehensive and responsible."

Abedi made 'farewell' call to his mother

After Abedi's mother and three of his siblings were brought in for questioning in Libya, a Libyan official told of a final phone call he placed to his mother and brother just before the attack.

On that call, he purportedly told his mother: "Forgive me," said Ahmed bin Salem, a spokesperson for the Special Deterrent Force.

"He was giving farewell," bin Salem added.

The Tripoli-based spokesperson for the Special Deterrent Force, an anti-terrorism unit, said that Hashim Abedi confessed both he and his brother were a part of ISIS and that Hashim had been aware of the details of the attack.

Salman Abedi's brother Hashim Ramadan Abedi appears inside the Tripoli-based Special Deterrent anti-terrorism force unit after his arrest on Tuesday for alleged links to the Islamic State extremist group. The force says that Hashim confessed both he and his brother were a part of ISIS and that Hashim had been aware of the details of the attack. (Ahmed Bin Salman/Special Deterrent Force/Associated Press)

Abedi had recently passed through Germany and Turkey, authorities said Thursday.

Four days before the attack, the British-born bomber "spent a short time in the transit area" of the Duesseldorf airport, according to police there, confirming an earlier report in the German magazine Focus.

Police didn't provide any further information or say where Abedi was coming from when he landed in the west German city.

Separately, a Turkish official told The Associated Press that Abedi travelled through Istanbul and Duesseldorf on his way to Britain.

Focus also reported that German authorities are trying to determine whether Abedi had contact with Islamic extremists in Germany. The report says British police informed their German counterparts that Abedi had received paramilitary training in Syria beforehand.

Greater Manchester Police said earlier two men were arrested overnight in Manchester and in the Withington area in the south of the city.

Ian Hopkins, the local police chief, told reporters the eight suspects in total who have been detained so far are "significant" arrests, and initial searches of premises have revealed items that police "believe are very important to the investigation." He didn't elaborate.

A woman was arrested late Wednesday but was later released without charge.

Officers also raided a property in the Moss Side area of the city early Thursday and carried out a controlled explosion.