LONDON (AP) — British police say everyone arrested over the Manchester concert bombing has been released without charge, but detectives are still not sure whether the attacker had accomplices.

Salman Abedi detonated a knapsack bomb as crowds were leaving an Ariana Grande concert on May 22, killing 22 people. He also died.

Shortly after the attack, police said they had rounded up members of Abedi's network. But Greater Manchester Police say all 22 suspects have now been released without charge.

Russ Jackson, head of counterterrorism policing for northwest England, said Sunday that police believe Abedi assembled the bomb himself, but it's unclear "whether he acted alone in obtaining the materials for the device."

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