LONDON -- All the suspects arrested over last month's Manchester concert bombing have been released without charge, British police said Sunday, acknowledging that detectives were still not sure whether the attacker had accomplices.

Salman Abedi, a Briton of Libyan heritage, detonated a backpack bomb as crowds were leaving an Ariana Grande concert on May 22, killing 22 people and himself.

Shortly after the attack, police said they had rounded up members of Abedi's network. But Greater Manchester Police said Sunday that all 22 people arrested on suspicion of terrorism offenses had been released without charge.

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Russ Jackson, head of counterterrorism policing for northwest England, said police believe Abedi assembled the bomb himself, but it was unclear "whether he acted alone in obtaining the materials for the device ... and whether others knew or were complicit in the storage of materials knowing what was being planned."

Jackson said some of those arrested had offered "accounts which explain innocent contact with Abedi." He said risk to the public had been considered before suspects were released.

Suspects arrested under terrorism laws can be held for up to 14 days before they must be charged or released.

Jackson said police had traced Abedi's movements in the weeks before the attack in detail, and "understand how the chemicals and equipment were obtained and where the bomb was assembled."

He said the vast police investigation would continue "as we work to understand the full extent of the involvement of anyone else."

Police released new images Sunday of Abedi waking through Manchester with a blue suitcase, which they believe contained bomb materials. Police were searching a landfill site for the case.

They also appealed for information from anyone who saw a white Nissan Micra that police believe Abedi used to transport and store bomb components.

Police also say they want to speak to Abedi's younger brother Hashem, who has been detained in Libya. That may prove difficult, however, as Libya has not had a stable central government since the ouster and killing of dictator Muammar Qaddafi in a Western-backed 2011 uprising.

A militia affiliated with the government in Tripoli, the Libyan capital, is holding Hashem Abedi, but it remains unclear to what extent that militia may be cooperating with the British investigation.