UK Freedom of Information Act

[FOXINBOX]

I do a lot of meandering around the internet, and as I am forced to be an armchair researcher, I rely on Open Source information. Now the UK has a Freedom of Information Act. You apply, say, for information on the murder of Eileen Pooley in 1968 i.e 52 years ago. It's a case in which the police not only found the offender, but took him to trial, the trial was reported in the national press, the guy was found guilty, sentenced, and the case was closed. Somebody (Not me) applied for information on the case under this law. They got zilch, the whole caboodle had been redacted. My question is, WHY? What possible harm could there be in revealing more details? They can still do a Keep Back on items of information that might cause distress to any surviving relations. Is it an administrative Get Out or have they literally lost the info?