Women cry after placing flowers in a square in central Manchester, Britain, Wednesday, May 24, 2017, after the suicide attack at an Ariana Grande concert that left more than 20 people dead and many more injured, as it ended on Monday night at the Manchester Arena. (AP Photo/Emilio Morenatti)
Women cry after placing flowers in a square in central Manchester, Britain, Wednesday, May 24, 2017, after the suicide attack at an Ariana Grande concert that left more than 20 people dead and many more injured, as it ended on Monday night at the Manchester Arena. (AP Photo/Emilio Morenatti) Credit: Emilio Morenatti

Manchester, England — The investigation into a suicide blast that killed at least 22 people at a pop music concert dramatically widened and intensified Wednesday, with security services on two continents rounding up suspects amid fears that the bombmaker who devised the bolt-spewing source of the carnage remains at large.

The arrests stretched from the normally quiet lanes of a northern English town by balaclava-wearing, plainclothes police officers to the bustling streets of Tripoli, where Libyan officials said they had disrupted a planned attack by the suspect’s brother.

But by day’s end, British authorities acknowledged that they remained vulnerable to a follow-up attack, with the nation’s state of alert stuck at “critical” — the highest possible level. The sight of soldiers deploying at London landmarks such as Buckingham Palace and 10 Downing Street underscored the gravity of a threat that was known in general terms before Monday night’s explosion but has come sharply into focus in the 48 hours since.

The morning after the attack, police had said that they believed the suspect, 22-year-old Salman Abedi, had carried it out alone and had died in the blast he triggered. But in their public statements Wednesday, authorities expressed growing confidence that Abedi — who had recently returned from Libya and may have also traveled to Syria — had been only one part of a significantly broader web of plotters behind Britain’s worst terrorist attack in more than a decade.

“It’s very clear that this is a network we are investigating,” Greater Manchester Chief Constable Ian Hopkins said.

Hopkins said police were moving quickly to disrupt it, carrying out raids across the city and arresting four people, including Abedi’s older brother, Ismail. A fifth suspect was later apprehended carrying “a suspicious package” in the town of Wigan, about 20 miles west of Manchester.

On Wednesday evening, authorities arrested a female suspect in Manchester, bringing to six the number of people detained in Britain in connection with the blast. The explosion claimed victims as young as 8 and targeted fans of American pop star Ariana Grande.

In Libya, counterterrorism authorities said they had arrested at least two additional members of Salman Abedi’s family, including a younger brother suspected of planning an attack in Tripoli.

Ahmed Dagdoug, spokesman for Libya’s counterterrorism Reda Force, said Hashem Abedi was arrested late Tuesday and is suspected of “planning to stage an attack in Tripoli.”

Abedi’s father, Ramadan, was arrested Wednesday, though it was not clear on what grounds. Ramadan Abedi had earlier asserted that his sons were innocent, telling the Associated Press that “we don’t believe in killing innocents. This is not us.”

He said Salman sounded “normal” when they last spoke five days ago. The elder Abedi said his son had planned to visit Saudi Arabia and then spend the Islamic holy month of Ramadan with family in Libya.