Manchester suicide bomber Salman Abedi did not act alone, police suspect, and detectives want to speak to his brother in Libya, anti-terror officers have said.
Detective Chief Superintendent Russ Jackson, head of the North West Counter Terrorism Unit (NWCTU), said officers were "engaged" with the authorities in war-torn Libya where Salman's brother, Hashem, is currently being held.
Mr Jackson also told reporters that, while detectives do not now think Abedi was part of a large network, they do suspect the involvement of others in the attack which had been planned for months.
And he did not rule out further arrests.
He said: "We don't have evidence of a large network. We do however suspect others were either aware or complicit in the knowledge of this attack."
Abedi killed 22 people when he detonated his bomb in the foyer of Manchester Arena at the end of an Ariane Grande concert on May 22.
Mr Jackson said he had to be careful not to damage any potential future prosecution as he briefed reporters at the headquarters of Greater Manchester Police (GMP).
He continued: "We do believe that there are other people potentially involved in this. We do however believe further arrests are possible.
"We want to to speak to Hashem Abedi and I can now say we are currently engaging with the Crown Prosecution Service and the Libyan authorities.
"This is a live criminal investigation where central to it are 22 murdered people, with grieving families."
Mr Jackson said attempts to speak to Hashem Abedi were part of an "ongoing" process but he refused to elaborate or say whether British police had travelled to Libya.
However, he did give some detail about what detectives had discovered so far.
Abedi had spent "several hours" in Manchester city centre with his rucksack bomb before heading for Manchester Arena, detonating the device as the concert ended and young fans streamed out.
Mr Jackson said police were now clear on Abedi's movements in the run-up to the attack, they knew the make-up of the bomb and where parts were obtained.
He would not confirm the exact type of explosive used but said forensic evidence had been found at several locations around Manchester.
He continued: "We are examining all sorts of lines of inquiry and it is possible more arrests and searches will take place.
"We do want to interview Hashem Abedi.
"This investigation is likely to run on for many more months to come."
The bomber's younger brother, Hashem, was arrested in Libya shortly after the explosion, along with their father, Ramadan.
The family are originally from Libya, but fled during Colonel Muammar Gaddafi's dictatorship, with the 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.
Mr Jackson said Salman 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", Mr Jackson said, but would not be drawn further.
He said no video or note has been found by police left by Abedi to explain his motivation.
Asked where Abedi got the money to live, travel and rent properties around Manchester prior to the attack, Mr Jackson said he had access to student loans and may have also had access to other funds, and that the planning took "many months".
An ongoing review by security services is under way into what was known about Abedi by the authorities, after claims that he had been reported more than once over his extremist attitude.
Mr Jackson said police are still piecing together details about the bomber's background and beliefs.
He added: "We are still working to understand the manner by which he became radicalised and this is a main part of our inquiry and it is difficult to comment further on that."