Three men arrested in connection with the Manchester attack have been released without charge as evidence mounts that Salman Abedi may have built the bomb detonated at Manchester Arena himself.

Greater Manchester Police said 11 suspects remain in custody as part of the probe into the suicide bombing that killed 22 victims.

Two men aged 20 and 24 from the Fallowfield area and a 37-year-old man from the Blackley area have been released.

Detective Chief Superintendent Russ Jackson, head of the North West Counter Terrorism Unit, said the release of further suspects "can be expected".

"We still have a number of people in custody and we will be seeking to extend the custody of some of them as we work to understand what has gone on and whether Abedi was helped," he added.

"There has been huge progress made over the week and the speed of the inquiry remains the same.

"It will be a long investigation and it will take considerable time before we fully understand what has happened."

Bomber Salman Abedi pictured moments before Manchester Arena massacre

Ariana Grande is to return to Manchester on Sunday alongside artists including Coldplay, Justin Bieber and Take That.

They will perform a tribute concert at Old Trafford, with all proceeds going towards the We Love Manchester fund set up for victims and their families.

Investigators have been attempting to piece together Abedi's movements between when he arrived back in Manchester from Libya on 18 May, and the bombing four days later.

The origin of the bomb, believed to have used homemade TATP explosives employed by Isis in the Paris and Brussels attacks, is of particular concern.

The Independent revealed last week that bomb-making materials were found during police searches, prompting concern over the potential construction of other devices.

Abedi's former landlord said a flat in Blackley had smelled of chemicals, telling investigators the suspected Isis supporter told him he was "flying abroad" six weeks ago, when he flew to Libya.

Manchester explosion in pictures Show all 37 1 /37 Manchester explosion in pictures Manchester explosion in pictures People running down stairs as they attempt to exit the Manchester Arena after a blast, where U.S. singer Ariana Grande had been performing, in Manchester Twitter/@ZACH_BRUCE/ via REUTERS Manchester explosion in pictures Helpers attend to people inside the Manchester Arena after a suspected suicide bomber detonated an explosive device at the end of an Ariana Grande concert, killing 22 people PA wire Manchester explosion in pictures Armed officers guard outside a hotel near the Manchester Arena following reports of an explosion, in Manchester, Britain EPA Manchester explosion in pictures Police officers are seen outside the Manchester Arena, where U.S. singer Ariana Grande had been performing, in Manchester, northern England Reuters Manchester explosion in pictures MANCHESTER, ENGLAND - MAY 23: Police escort members of the public from the Manchester Arena on May 23, 2017 in Manchester, England. An explosion occurred at Manchester Arena as concert goers were leaving the venue after Ariana Grande had performed. Greater Manchester Police have confirmed 19 fatalities and at least 50 injured. (Photo by Christopher Furlong/Getty Images) Getty Images Manchester explosion in pictures MANCHESTER, ENGLAND - Emergency services arrive close to the Manchester Arena on May 23, 2017 in Manchester, England. There have been reports of explosions at Manchester Arena where Ariana Grande had performed this evening. Greater Manchester Police have have confirmed there are fatalities and warned people to stay away from the area. (Photo by Dave Thompson/Getty Images) Getty Images Manchester explosion in pictures MANCHESTER, ENGLAND - MAY 23: Police escort members of the public from the Manchester Arena on May 23, 2017 in Manchester, England. An explosion occurred at Manchester Arena as concert goers were leaving the venue after Ariana Grande had performed. Greater Manchester Police have confirmed 19 fatalities and at least 50 injured. (Photo by Christopher Furlong/Getty Images) Getty Images Manchester explosion in pictures Medics deploy at the scene of a reported explosion during a concert in Manchester, England on May 23, 2017. British police said early May 23 there were "a number of confirmed fatalities" after reports of at least one explosion during a pop concert by US singer Ariana Grande. Ambulances were seen rushing to the Manchester Arena venue and police added in a statement that people should avoid the area / AFP PHOTO / Paul ELLIS (Photo credit should read PAUL ELLIS/AFP/Getty Images) AFP/Getty Images Manchester explosion in pictures Police and fans close to the Manchester Arena, after reports of an explosion Getty Images Manchester explosion in pictures There have been reports of explosions at Manchester Arena where Ariana Grande had performed Getty Images Manchester explosion in pictures Police deploy at scene of explosion in Manchester, England, at a concert in Manchester Arena AFP/Getty Images Manchester explosion in pictures Police stand by a cordoned off street close to the Manchester Arena Getty Images Manchester explosion in pictures Police deploy at scene of explosion in Manchester, England AFP/Getty Images Manchester explosion in pictures Police deploy at scene of a reported explosion during a concert in Manchester, England, on May 23, 2017. British police said early May 23 there were "a number of confirmed fatalities" after reports of at least one explosion during a pop concert by US singer Ariana Grande. Ambulances were seen rushing to the Manchester Arena venue and police added in a statement that people should avoid the area AFP/Getty Images Manchester explosion in pictures An ambulance drives away from the Manchester Arena, where U.S. singer Ariana Grande had been performing, in Manchester Reuters Manchester explosion in pictures Police escort members of the public from the Manchester Arena in Manchester, England. Getty Images Manchester explosion in pictures A woman sits in the street in a blanket near the Manchester Arena as police guard the area following reports of an explosion, in Manchester, Britain EPA Manchester explosion in pictures Two women wrapped in thermal blankets stand near the Manchester Arena, where US singer Ariana Grande had been performing, in Manchester Reuters Manchester explosion in pictures A Royal Logistic Corps (RLC) bomb disposal robot is unloaded outside the Manchester Arena following reports of an explosion, in Manchester. At least 19 people have been confirmed dead and others 50 were injured, authorities said. It is being treated as a terrorist incident until police know otherwise EPA Manchester explosion in pictures A Royal Logistic Corps (RLC) bomb disposal robot is unloaded outside the Manchester Arena following reports of an explosion, in Manchester EPA Manchester explosion in pictures Members of the public receive treatment from emergency service staff at Victoria Railway Station close to the Manchester Arena on May 23, 2017 in Manchester, England. There have been reports of explosions at Manchester Arena where Ariana Grande had performed this evening. Greater Manchester Police have have confirmed there are fatalities and warned people to stay away from the area Getty Images Manchester explosion in pictures Armed police after a suspected terrorist attack at the Manchester Arena at the end of a concert by US star Ariana Grande left 19 dead PA wire Manchester explosion in pictures Emergency services arrive close to the Manchester Arena in Manchester Getty Images Manchester explosion in pictures An amoured police vehicle patrols near Manchester Arena in Manchester Getty Images Manchester explosion in pictures A man carries a young girl on his shoulders near Victoria station in Manchester AFP/Getty Images Manchester explosion in pictures Police officers stand at the Miller Street and Corporation Street Crossroads, in front of the Manchester Arena in Manchester, England Getty Images Manchester explosion in pictures Police block a road near to the Manchester Arena in central Manchester, England AP Manchester explosion in pictures Armed police patrol near Victoria station in Manchester, northwest England. Twenty two people have been killed and dozens injured after a suspected suicide bomber targeted fans leaving a concert of US singer Ariana Grande in Manchester Getty Images Manchester explosion in pictures Police forensic officers leave the Manchester Arena as they investigate the scene of an explosion in Manchester Getty Images Manchester explosion in pictures A forensic officer collects evidence on a walkway between Victoria station and Manchester Arena following a deadly terror attack in Manchester, Getty Images Manchester explosion in pictures A woman and a young girl wearing a t-shirt of US singer Ariana Grande talks to police near Manchester Arena following a deadly terror attack in Manchester, Getty Manchester explosion in pictures epaselect epa05982725 Susan Walton and daughter Katie, 10 (pictured), who attended the concert of Ariana Grande at the Manchester Arena, are seen in Manchester, Britain, 23 May 2017. According to a statement released by the Greater Manchester Police on 23 May 2017, police responded to reports of an explosion at Manchester Arena on 22 May 2017 evening. At least 19 people have been confirmed dead and around 50 others were injured, authorities said. The happening is currently treated as a terrorist incident until police know otherwise. According to reports quoting witnesses, a mass evacuation was prompted after explosions were heard at the end of US singer Ariana Grande's concert in the arena. EPA/NIGEL RODDIS EPA Manchester explosion in pictures Mayor of Greater Manchester Andy Burnham and Manchester City Council Leader Sir Richard Leese speak to the media outside Manchester Town Hall after a suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester Dave Higgens/PA Wire Manchester explosion in pictures The media gather behind a police cordon in Manchester Getty Images Manchester explosion in pictures Flowers left close to the Manchester Arena, the morning after a suicide bomber killed 22 people, including children, as an explosion tore through fans leaving a pop concert in Manchester Danny Lawson/PA Wire Manchester explosion in pictures Ariana Grande concert attendees Karen Moore and her daughter Molly Steed, aged 14, from Derby, leave the Park Inn where they were given refuge after last night's explosion at Manchester Arena Getty Manchester explosion in pictures Signs saying 'We love Manchester' are displayed in a window in Manchester, England Getty Images

A metal rod, traces of chemicals and squares of cut-up material were found at the flat, while unused chemicals were found at Abedi's former home in Fallowfield.

The bomber is believed to have made his final preparations in another flat in Granby Row, central Manchester, where he spent his final hours.

Det Ch Supt Jackson confirmed that Abedi himself had made "most of the purchases of the core components", following reports he had visited DIY shops in Manchester.

"What is becoming apparent is that many of his movements and actions have been carried out alone during the four days from him landing in the country and committing this awful attack," he added.

"It is vital that we make sure that he is not part of a wider network and we cannot rule this out yet.

"There remain a number of things that concern us about his behaviour prior to that attack and those of his associates."

The detective said investigators were especially keen to find out why Abedi repeatedly returned to the Wilmslow Road area of Manchester, and want to recover a blue suitcase used in those trips.

Police released an image of the bomber carrying a distinctive blue suitcase (AP)

More than 1,000 police officers have worked on the case, examining 7,000 lines of inquiry and 300 phones and other digital equipment, with 18 scenes still under guard.

Police have been tracking Abedi's last movements using CCTV, phone calls and other data exposing his interactions, amid questions over how he slipped through the net.

The 22-year-old had a “relatively minor” criminal record as a teenager, when he was linked to gangs, smoked cannabis and drank, but he was not known to police for holding extremist views.

He appeared on police logs in 2012 over offences of theft, receiving stolen goods and assault and was known to MI5, which has launched an internal review amid reports it was warned about Abedi's plans.

Abedi had been reported to the counter-terror hotline and banned from a local mosque, but was not known to Prevent, the Government's voluntary counter-radicalisation scheme.

Among the suspects still in custody are some of Abedi's friends and relatives, and a Libyan trainee pilot arrested in Shoreham-by-Sea.

It is understood the number of people considered previously by counter-terror teams runs to around 20,000, in addition to around 3,000 individuals flagged up as part of 500 live investigations currently being pursued.