Terrorism investigators have confirmed that they believe the Manchester Arena suicide bomber, Salman Abedi, did not act alone when preparing his attack on an Ariana Grande concert that killed 22 people.

In a briefing to the media on Thursday, investigators said that others may have been complicit in helping Abedi. Counter-terrorism officers have said that accomplices of the 22-year-old college student were aware that he was about to carry out the deadliest terror attack on UK soil since the 7/7 bombings.

Investigators revealed that although Abedi was not part of a terror network, there were others who knew he was planning to detonate a substantial nail bomb at the Ariana Grande concert.

Following the revelations, counter-terrorism officers repeatedly told a packed media conference that they were seeking to speak to Abedi’s younger brother, Hashem, 20. They have contacted Libyan authorities, but it was insinuated that they were anticipating lengthy complications before a request for access to Hashem was granted.

Hashem was arrested in Libya shortly after the explosion on 22 May, along with their father, Ramadan. Both are being held by the Special Deterrence Force, also called Rada.

In an interview with Reuters, Ahmed bin Salem, Rada’s spokesman, previously said Hashem helped his brother to buy bomb-making equipment in Britain.

During the briefing DCS Russ Jackson, head of north-west counter-terrorism policing, said that others were involved and there could still be further arrests.

He said: “We do believe that there are other people potentially involved in this. We do believe further arrests are possible.

“We are currently engaging with the Crown Prosecution Service and the Libyan authorities,” he said. “This is a live criminal investigation where central to it are 22 murdered people with grieving families.”

In a chilling account of the hours prior to the attack, Jackson revealed that Abedi had spent several hours “milling around” the busy city centre before eventually deciding to detonate his bomb at the concert.

He said: “There were a number of opportunities he would have had to detonate the device.

“On that evening, we’ve got him at different points in Manchester. That’s not all going to crowded places, but he’s got it in public places, at different points, with the backpack on.

“If we assume it’s all primed and ready to go – and that would be a reasonable working assumption – there were different points he could have done it.

“He’s clearly gone to different locations. We’re never going to get into this man’s mind. Our assessment is that he’s targeted the Arena. He wasn’t looking for somewhere else that evening.”

New details also emerged of the devastation caused by the bomb, packed inside a tin with shrapnel including nuts and bolts surrounding it.

When the device was detonated it was so powerful it caused a crater in the arena’s concrete floor and a 23 sq metre (250 sq ft) glass skylight shattered.

Jackson also revealed the impact on officers who worked during the attack. He said that forensic officers were in tears after recovering victims’ blood-spattered phones with photographs of children on them.

The explosion ripped through the arena foyer as thousands of fans, mostly young girls and their parents, were leaving the venue after the performance.

Police previously said they believed Abedi assembled the device by himself in the days before the attack.

They had said it was unclear whether he acted alone in obtaining bomb materials, which officers believed were stored in a white Nissan Micra found parked in the Rusholme area of the city.

But they have now confirmed that others were aware of his plans, and said they were looking into his connections with other Manchester terrorists, including Isis recruiter Raphael Hostey. They also confirmed that he twice visited convicted terrorist Abdal Raouf Abdallah in prison.

Abdallah, left paralysed after being shot while fighting against the Gaddafi regime in the 2011 uprising in Libya, was jailed for five years for trying to help people travel to Syria to join jihadis. Abedi visited Abdallah twice, in January and March 2017 at HMP Altcourse in Liverpool.

Abedi was born in Manchester to Libyan parents, who moved back to Tripoli in recent years, along with his younger siblings. He is believed to have visited Libya shortly before the attack, arriving back in the UK on 18 May.

The family had fled Libya during Col Muammar Gaddafi’s dictatorship, with Abedi’s father returning to fight with opposition forces when the uprising began in 2011.

Abedi’s older brother, Ismail, was among more than a dozen people held and questioned by police in the UK before being released without charge. Police have not ruled out the possibility of rearresting those they have already questioned.

Jackson said Abedi travelled to Libya a number of times and they were investigating how he obtained the skills to make a bomb.

Calls Abedi made, reportedly to his mother and others, on the night of the attack were another key line of inquiry, Jackson said, but he would not be drawn further.

Jackson added: “

The searches that we conducted on average took eight days to complete. The digital exhibits we’ve seized contain over 3 million files and 15 terabytes. We have now nearly 16,000 hours of CCTV and we anticipate it will take many months to work through this.

“We have taken 755 statements and there are 3,200 people who form part of the investigation.”