Back in 2008, one of the PMF people, who called himself Damian, had a blog on which he translated Italian media coverage of the Kercher murder investigation and pre-trial. He translated an October 19, 2008 article in Messaggero Umbria, summarizing an interview in which Mignini described his theory of the crime. It included the following:
...what’s more, the three, and especially Sollecito, were very susceptible to ‘cultural’ influences, erotic-murders which have already been spoken about. That night was the feast day of the Saints, the Catholic ‘heir’ to the Celtic new-year Samhain, with all the implications which are focused on the eve of the feast day, that is the night between Oct 31 and Nov 1.
On October 20, La Nazione reported the following, as translated by Damian:
Meredith could have died on the evening of Oct 31st because that night was favourable for a ’sexual rite’ in accordance with the traditions of Halloween. For this reason, according to the police, Amanda, who is believed to be both the instigator of the crime and the person who carried it out, was looking for Meredith and sent her numerous sms messages. [ ... ]
However, the house wasn’t free - the young Italian women were having dinner there - and Meredith spent the whole evening with her English friends. “Amanda (and Raffaele) tried to meet Meredith on the evening of Halloween and this explains their position of observation on the steps near via della Pergola overlooking the house, as described by Curatolo,” says the public prosecution.
At this time, Mignini was trying to explain away the problems in Curatolo's testimony by claiming the junkie must have seen Amanda and Raffaele on both Halloween and the night of the murder. But Amanda was able to prove she was elsewhere on Halloween, and Mignini eventually dropped this idea.
In any case, this is the origin of the satanic theory reported by various UK outlets. Mignini's defenders were quick to point out that it is not precisely satanic. Fair enough. It is still a crackpot fantasy with no basis in fact.
There is no doubt Mignini (along with other investigators) proposed a satanic cult conspiracy to explain the Monster of Florence killings. FBI experts and others who have looked at these cases believe they were the work of a necrophiliac who cut away the genitals of his female victims and removed them from the crime scene so he could use them for his fetish activity. This is a rare type of criminal behavior, but one with well-known precedents.
Mignini believed the murders were commissioned by a satanic cult, and he proposed a theory that has no precedents. He speculated that the membership of this cult, which has never been proven to exist, included some of the leading citizens of Florence. According to Mignini, they purchased the severed genitalia for use in their rituals. That is how Mignini ended up asking for more than 20 indictments following his investigation of the Narducci drowning, and it is why he tapped the phones of his colleagues in Florence. He thought Narducci was a cult member who was killed because he had become a security risk, and he thought investigators in Florence were covering up the conspiracy.