The suicide bomber in Manchester called his mother to ask for forgiveness hours before he carried out the attack at an Ariana Grande concert, an anti-terror investigator in Libya revealed Thursday.

Salam Abedi, 22, called his mother and said, "Forgive me," Special Deterrent Force spokesman Ahmed bin Salem said. Abedi's mother and three siblings in Libya were summoned for questioning.

"He was giving farewell," bin Salem said, adding that Libya investigators believe the bomber acted alone based on what his father, Hashim Abedi, told them.

Other terror links involving the bomber's relatives emerged Wednesday. Abedi's father, Ramadan, was a member of the Al Qaeda-backed Libyan Islamic Fighting group in the 1990s, according to former security official Abdel-Basit Haroun -- but the father denied the claim.

In addition, the killer's brother Hashim had links to the Islamic State and may have been planning a separate attack in Tripoli, a Libyan government spokesman told Reuters.

Salman Abedi left Libya for England four days before the bombing, his mother said. He apparently fooled his parents by telling them he was heading to a pilgrimage in the Muslim holy city of Mecca, The Wall Street Journal reported.

Among his stops: Turkey and Germany, though he appeared to make transfers at airports instead of staying in those countries.

Abedi flew into Istanbul on May 18 and later departed for Dusseldorf, a Turkish official told The Associated Press. The official did not say where the flight had departed before landing in Istanbul, but pointed out the attacker had on several occasions in the past used Istanbul as a transit for flights between Libya and Europe.

German officials suggested he transferred in Dusseldorf as well, Sky News reported. Abedi did not appear to be on an international terror watch list, which allowed him to travel freely throughout Europe, according to the news agency.

British Transport Police also announced that armed officers would patrol some U.K. trains for the first time starting Thursday afternoon because of the increased threat of extremist attacks. British Transport Police Chief Constable Paul Crowther said the force has "radically increased" its presence since the Monday night attack.

Authorities raided an apartment on Wednesday believed to be the place where Abedi visited before the attack. The apartment is advertised on a rental website as an "ideal" place to stay next to the Manchester arena, according to Sky News. Abedi visited the apartment about 7 p.m. Monday, three hours before he detonated the bomb. It was unclear if Abedi was staying at the apartment in the days leading up to Monday.

Police were racing to uncover the network around Abedi that they said “clearly” exists. British investigators earlier said they believed the 22-year-old bomber was given the explosives used to kill himself and others at the pop concert. However, a source told Reuters Thursday that Abedi could have created the bomb himself.

The New York Times released photos of bomb parts found at the scene. Though they do not reveal the size or type of the device, the photos showed it was carefully created to make an impact.

Greater Manchester Police said two more men were arrested early Thursday. Eight men in total have been detained in connection with the attack, in what police have called “significant” arrests. A woman was arrested late Wednesday but was later released with no charges.

Officers raided a property early Thursday morning in the Moss Side of the city and carried out a “controlled explosion.”

Britain’s terrorism threat level has been raised to “critical,” meaning a new attack could be imminent.

The Islamic State has claimed responsibility for the attack, calling Abedi a “soldier of the caliphate.”

Abedi was believed to have traveled to Syria and had “proven” links with ISIS, French Interior Minister Gerard Collomb said Wednesday on BFM television, adding that British and French intelligence have information that Abedi had been to Syria.

A family friend told the Wall Street Journal that Abedi grew "increasingly religious" after his friend, Abdul Wahab Hafidah, was run over by a car and fatally stabbed in May 2016. Abedi viewed it as a hate crime and vowed for revenge, according to the friend.

British soldiers have been deployed in place of police officers to guard high-profile sites such as Buckingham Palace and Parliament.

The Associated Press contributed to the report.