Salman Abedi, the suicide bomber who killed 22 people at a pop concert in northern England last month, likely built the device he used himself in the days immediately before the attack, British police said.

Police said Abedi, a 22-year-old born in Britain to parents of Libyan birth, travelled to Libya on April 15 and returned on May 18, four days before he targeted the Ariana Grande show in Manchester.

Closed-circuit television footage showed him purchasing items for the bomb soon after he returned to Britain, including nuts from a hardware store that were used as shrapnel as well as the tin that was believed to contain the explosives, police said.

Detectives said they believed materials used for making the bomb were stored in a white Micra car that Abedi purchased on April 15, shortly before he flew to Libya.

"The actual assembly of all the parts appears to have been in the days immediately before the attack," Russ Jackson, head of North West Counter Terrorism Policing, said in a statement. "Our enquiries show that the assembly of the device is likely to have been by Abedi himself."

Police said they still needed to establish whether Abedi acted alone in obtaining and storing all of the materials, and whether others were aware of his plan. Abedi's brother Hashem left Britain at about the same time and was arrested along with the men's father Ramadan by Libyan counterterrorism investigators after the attack.

A spokesman for Tripoli's Special Deterrence Force told Reuters last week that Hashem Abedi had told them his brother was radicalized in Britain in 2015 and that he had bought the materials for Salman.

"Hashem Abedi is currently detained in Libya. There is much media reporting about what he has said in Libya and we wish to interview him in relation to this attack," Jackson said.