MANCHESTER, England -- However tragic the Manchester Arena bombing was, the terrorism investigation at least seemed straightforward at the beginning. Police said the bomber had acted alone.

By Wednesday morning, however, CBS News correspondent Mark Phillips says British authorities were no longer sounding so sure. Hours later, Greater Manchester Police Chief Constable Ian Hopkins confirmed that it was, in fact, "a network we are investigating."

Hopkins did not provide further detail, but police raids in the area around the south Manchester home of Salman Abedi had already led to increasing suspicions that Abedi was no lone wolf.

Hopkins confirmed that three people were taken into custody on Wednesday morning in the same south Manchester area, known as Moss Side. He also confirmed the arrest the previous day of a 23-year-old man in the same neighborhood.

Abedi was known to British authorities, but had been considered a minor figure -- on the fringes of Islamic militant movements, not an imminent threat.

He was born and raised in Manchester by parents who left Libya in the 1990s.

It has now emerged that Abedi traveled back to his ancestral homeland recently and security officials suspect he may also have traveled to Syria.

As Phillips reports, he may have been more directly connected to the Islamic State of Iraq and Syria (ISIS) than they had thought. The terror group claimed responsibility for the attack, which left 22 people dead, in a generic statement on Tuesday.

There were also reports Wednesday suggesting a possible al Qaeda link, with former FBI special agent Ali Soufan, who now runs a private security consultancy, tweeting a document purportedly showing Abedi's father's name on a document linking him to the terror group.

Ramadan Abedi, the bomber's father, has spent a lot of time in his native Libya in recent years. He told The Associated Press on Wednesday, from Tripoli, that his son was innocent, and that his family are not "the ones who blow up ourselves among innocents."

A former Libyan security official told the AP earlier Wednesday that he knew Ramadan Abedi to have been a member of the Libyan Islamic Fighting group before he fled to Britain in the 1990s. That group had links to al Qaeda, but it disintegrated years ago.

The former official told the AP that the elder Abedi remained, however, a follower of the strict Salafist interpretation of Islam espoused by both al Qaeda and ISIS.

The attack on the roughly 20,000 young Ariana Grande concertgoers appears to have come just a few weeks after Abedi returned from his last trip, and security forces now suspect a connection.

ISIS' de-facto capital and one remaining urban stronghold is in Raqqa, Syria, and the terror group also has a significant presence in northern Libya.

Salman Abedi, the Manchester native who blew himself up after an Ariana Grande concert at the Manchester Arena on May 22, 2017, is seen in an undated file photo. The Guardian

The area of Manchester that Abedi comes from, and where police have been operating, has been known as a recruiting ground for ISIS, with a dozen or so of its young people going off to fight in Syria.

The police raids in south Manchester and other information from the investigation led British Prime Minister Theresa May to raise the national terror threat to its highest warning level of "critical," indicating that authorities believe another attack could be imminent.

It is the first time the threat level has been raised to the critical level in the U.K. since 2007, after the deadly attack on London's Underground train network and buses.

"This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations," May told the rattled British public in a television address. "You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe."

British media reports suggested almost 1,000 troops were being deployed to sensitive sites across the country.

The impact will be felt by tourists in London, where the daily Changing of the Guard ceremony at Buckingham Palace, which draws thousands of onlookers, was cancelled on Wednesday. So too were all public tours of the Houses of Parliament.

There wasn't much in way of visible security to be seen on the streets as Mancunians got back to work on Wednesday, notes Phillips, but their sense of living in a secure city, perhaps always something of a delusion, is gone.