A 15-year-old boy was arrested at a school over the murders of two policewomen killed in a gun and grenade attack.
The teenager was held on suspicion of assisting an offender and questioned by police investigating the deaths of Pc Nicola Hughes, 23, and Pc Fiona Bone, 32, and father and son David and Mark Short.
He was later released on bail pending further inquiries.
The officers died on Tuesday after they were called to a report of a burglary in Hattersley, Greater Manchester, while Mr Short and his son were murdered in separate incidents earlier this year in Clayton and Droylsden.
Greater Manchester Police said officers attended a school in the Hattersley area on Friday afternoon.
Assistant Chief Constable Garry Shewan said: "This remains a complex and sensitive operation and we continue to ask the public to search their conscience and urge anyone who has any other information to contact us."
Meanwhile, earlier on Friday there was a massive security operation for the first court appearance of Dale Cregan, who is accused of the four murders and also four attempted murders.
The hearing for one-eyed Cregan before a district judge lasted just two minutes as he was remanded in custody and taken to Manchester's Strangeways prison.
Cregan, 29, sat down as he was brought into the dock at Manchester Magistrates' Court but got to his feet after he was asked to stand by District Judge Jonathan Taaffe.
He wore a false black onyx eye in his left eye socket and spoke only to confirm his name and date of birth to the courtroom, which was packed with reporters.