British police on Saturday released surveillance-camera images of the Manchester concert bomber on the night of the attack and asked the public for more information on his whereabouts leading up to bombing.

The photos released by police show attacker Salman Abedi on the night of the bombing, wearing sneakers, jeans, a dark jacket and a baseball cap. The straps of a knapsack are visible on his shoulders.

Greater Manchester Police chief Ian Hopkins and Neil Basu, the national coordinator of counterterrorism policing, urged people to contact police if they had information about Abedi's movements between May 18 and Monday night.

"In the past five days, we have gathered significant information about Abedi, his associates, his finances, the places he had been, how the device was built and the wider conspiracy," they said in a statement.

"Our priorities are to understand the run-up to this terrible event and to understand if more people were involved in planning this attack."

Abedi, a 22-year-old Briton of Libyan descent, died in the explosion, which killed 22 others and wounded nearly 120 as crowds were leaving an Ariana Grande concert.

Police said Saturday he returned to Britain before the attack. His father has said Abedi was in Libya until earlier this month and had told family he had planned to go to Saudi Arabia on a pilgrimage.

Police say they think Abedi assembled his bomb at a rented apartment in central Manchester that was raided by officers Wednesday.

British Prime Minister Theresa May said "a significant amount of police activity" and several arrests had led to the level being lowered. But she urged Britons to remain vigilant and said soldiers would remain at high-profile sites throughout the weekend, and start reducing their presence beginning Tuesday.

A severe threat still means an attack is "highly likely," according to the scale set by Britain's Joint Terrorism Analysis Centre.

Assistant Commissioner Mark Rowley, Britain's top counterterrorism police officer, said authorities have dismantled a "large part" of the network around the bomber.

But Rowley said there were still "gaps in our understanding" of the plot, as investigators probed Abedi's potential links to jihadis in Britain, Europe, Libya and the Middle East.

"There will be more arrests and there will be more searches," he said.

Police made two more arrests in Manchester on Saturday on suspicion of terrorism offenses, bringing the number of suspects in custody to 11. All are men, aged between 18 and 44. In addition, Abedi's father and younger brother were detained in Libya.

The Associated Press contributed to this report.