Salman Abedi was particularly upset by the killing of a Muslim friend last year, and he believed the death went unnoticed by “infidels” in the U.K.

The alleged culprit in the deadly Manchester concert bombing was driven by what he saw as unjust treatment of Arabs in Britain, a relative has said, confirming he made his final phone call in which he pleaded: “Forgive me.”

Salman Abedi was particularly upset by the killing of a Muslim friend last year, and he believed the death went unnoticed by “infidels” in the U.K., said the relative on Thursday, speaking on condition of anonymity over concerns for her own security.

“Why was there no outrage for the killing of an Arab and a Muslim in such a cruel way?” she asked. “Rage was the main reason,” for the blast that killed 22 at the end of an Ariana Grande concert at Manchester Arena on Monday, she said, speaking through telephone from Libya.

The new insight into Abedi’s motivation came as Britons faced stepped up security, authorities pushed forward with raids and the investigation extended across Europe into Libya, where most of the suspected bomber’s family lived.

The number of arrests in the U.K. ticked up to eight as British Transport Police said armed officers would begin patrols on some trains because of an increased threat of terrorism. Greater Manchester Police Chief Constable Ian Hopkins said, without elaborating, that searches of suspects’ homes brought “very important” clues in the probe into the bombing. But leaks from the investigation were creating a trans-Atlantic diplomatic mess.

The Manchester police halted their sharing of investigative information with the United States through most of the day until receiving a fresh promise that there would be an end to leaks to the media.

British Prime Minister Theresa May, who spoke about the matter with U.S. President Donald Trump at a NATO summit in Brussels, said the countries’ partnership on defence and security was built on trust. But “part of that trust is knowing that intelligence can be shared confidently,” she noted.

Mr. Trump pledged to “get to the bottom” of the leaks, calling them “deeply troubling” and asking the Justice Department and other agencies to “launch a complete review of this matter.”

Emergency services work at Manchester Arena after the explosion at the venue during an Ariana Grande concert in Manchester. Police advised the public to avoid the area around the venue. A representative said the singer was not injured. A person is wheeled away on a stretcher at Victoria Railway Station close to the Manchester Arena. Emergency vehicles were on the scene helping the injured and bomb disposal units were later seen outside the venue. Members of the public are escorted from the Manchester Arena to a safer place. A number of Manchester taxi services said they were offering free rides to people trapped by the incident. Members of the public receive treatment from emergency service staff at Victoria Railway Station close to the venue. Armed police stand next to an ambulance at the Manchester Arena. Greater Manchester Police say they are working with national police and intelligence agencies in what is being treated as a terrorist incident. Police block roads near to the Manchester Arena, seen at the right, in central Manchester, on Tuesday. City officials said the true spirit of Manchester was surfacing in the hours after the incident. Many Manchester residents responded early on Tuesday with offers of shelter and details on locations where displaced concert-goers had been taken in. A police officer with a sniffer dog patrols near the Manchester Arena early on Tuesday. Police said on Tuesday morning they are still gathering information about the incident and are setting up a telephone hot line to help people locate loved ones.

British officials were particularly angry over photos published by The New York Times showing remnants of a blue backpack that may have held the explosive. But it wasn’t clear if U.S. officials were the source of the images, which the Times defended as “neither graphic nor disrespectful of victims” and consistent with basic reporting “on weapons used in horrific crimes.”

British security services were also upset that 22-year-old Abedi’s name was apparently leaked by U.S. officials while police in the U.K. continued withholding it and while raids were on in Manchester and in Libya. Mr.Hopkins said the leaks “caused much distress to the families that are already suffering terribly with their loss.”

Meanwhile, the investigation into the blast widened.

The authorities chased possible links between Abedi and militants in Manchester, elsewhere in Europe, and in North Africa and the Middle East. They were exploring potential ties to Abdalraouf Abdallah, a Libyan jailed in the U.K. for terror offences, and to Raphael Hostey, an Islamic State recruiter killed in Syria.

Abedi’s family remained in focus too, with a brother in England, his father and another brother in Libya among those detained. Abedi’s father was allegedly a member of the al- Qaeda-backed Libyan Islamic Fighting group in the 1990s, a claim he denies.