May 28 (UPI) -- British police said they have 13 suspects in custody after their 15th arrest Sunday in connection with the Manchester, England, suicide bombing that killed 22 last week.
Police arrested an unidentified man, 25, in the southwest of Manchester "on suspicion of offenses contrary to the terrorism act," Greater Manchester Police posted on Twitter.
British police also apprehended a 19-year-old man during a raid of the former home of the Salman Abedi, the bombing suspect. Of the 15 arrests thus far, two men have been released without charges and 13 others remain in custody on suspicion of terror offenses.
Home Secretary Amber Rudd warned Sunday on BBC that members of the network behind the attack by the deceased Abedi remain "potentially" unaccounted for, but "the operation is still really at full tilt in a way."
Also Sunday, MI5, Britain's domestic counter-intelligence and security agency, acknowledged an inquiry into how it handled warnings from the public about Abedi. MI5 was alerted to his extremist views at least three times.
On Saturday, Prime Minister Theresa May lowered the threat level from its highest rating "critical," meaning an attack could be imminent, to "severe."
British police also released a new photo Saturday of Abedi taken from security footage on the night of the deadly attack.
National Health Service said 54 people injured in the attack after the conclusion of Ariana Grande's concert at Manchester Arena were still being treated in eight hospitals, including 19 in critical care.
A total of 116 people required hospital care, police said.
The Islamic State took responsibility for the attack.
Last week, the Royal Air Force scrawled the message "love from Manchester" on a Paveway IV bomb meant for airstrikes against the Islamic State.
An RAF spokesperson confirmed to the Telegraph that the photo was genuine.
"The sentiment of the message written on the weapon is understandable and such writing has history in the RAF, so the individual concerned will not be taken to task," a source was quoted as saying.
Australian Sen. James McGrath posted the image on Facebook.