The investigation into the Manchester Arena bombing has so far cost £4m, with 7,000 people now referenced in the inquiry, Greater Manchester police (GMP) have said.

The force provided an update on its investigation as it was awaiting the extradition of the bomber’s younger brother from Libya. No one has yet been charged with helping Salman Abedi kill 22 people in a suicide attack at the end of an Ariana Grande concert on 22 May last year.

The GMP assistant chief constable Russ Jackson said he now had a “deep understanding of the actual events that happened and the motivations”.

Twenty-three people were arrested in the days and weeks after the attack and all were released without charge. No one has since been interviewed under caution in relation to the case. Jackson said 2,000 witness statements had been taken.

He said he remained grateful to the Libyan authorities for considering a request from the Crown Prosecution Service (CPS) last October to extradite Hashem Abedi, the bomber’s brother, who has been held by a Libyan militia since two days after the attack. If he is returned to the UK, he will be charged with the murder of 22 people, the attempted murder of others who were injured, and conspiracy to cause an explosion.

Around 100 officers are still working full time on the investigation. Two officers have spent the past year returning items found at the arena to concertgoers and bereaved families. Until last week another team was searching through a rubbish tip next to the M66 between Bury and Heywood in Greater Manchester, looking for a blue suitcase that Abedi had transported around Manchester in advance of the attack.

After combing through 11,000 tonnes of rubbish, the search had proved fruitless, GMP said on Tuesday. Jackson said the investigation so far had cost £4m from the counter-terrorism budget.

The force believes 800 people were physically or psychologically harmed as a result of the attack – a big increase on its previous estimates. “We are consistently moved by the grace and dignity they show in trying to repair their lives,” Jackson said. “Of course, for many the loss is too great for them to ever make a full recovery from this terrible event.”