[1] Several connected persons have been convicted for involvement in the murders, yet the exact sequence of events, the identity of the main actor and the motives remain unclear.

Multiple weapons were used in the murders, including a .22 caliber Beretta handgun and a knife, and in half of the cases a large portion of the skin surrounding sexual organs was excised from the bodies of the female victims.

On the night of 21 August 1968, mason worker Antonio Lo Bianco (29) and homemaker Barbara Locci (32) were shot to death with a .22 handgun in Signa, a small town west of Florence.

[2] Authorities were unable to reconstruct the chain of custody of those pieces of evidence and did not request a scientific comparison, even though it would have been necessary to check whether they matched the ballistic report from 1968.

As the spent cartridges were fired by a gun used in four similar crimes, their presence in the Mele's case file suggested to law enforcement officers that the perpetrator of the more recent double murders was connected with them.

On 6 June 1981 warehouseman Giovanni Foggi (30) and shop assistant Carmela De Nuccio (21) were shot and stabbed near Scandicci, where the engaged couple both lived.

On the night of 7–8 September 1985 Jean Michel Kraveichvili (25), a musician of Georgian ancestry, and tradeswoman Nadine Mauriot (36), both from Audincourt, France, were shot and stabbed while sleeping in their small tent in a woodland area near San Casciano.

By this point, chief prosecutor Pier Luigi Vigna thought the Sardinian trail was spent, and wanted to look into the possibility of the gun having been picked up by an unknown party after its use in the 1968 murder.

Pacciani had been convicted both for rape and domestic abuse of his two daughters, and for the 1951 murder of a man who had relations with his ex-girlfriend, for which he served thirteen years in prison.

Inspector Ruggero Perugini found incriminating evidence, such as similarities between the 1951 murder and the Monster killings, as well as a reproduction of Primavera by Botticelli and another painting thought to be by Pacciani.

[12] Vanni had been a witness at Pacciani's trial, where he famously claimed the two of them merely to be "Snack Buddies" (Compagni di Merende), a term that entered Italian vernaculum.

[16] The acquaintances of Pacciani and Vanni during the years of the murders fueled a line of investigation into possible esoteric motives and rites linked to satanism underlying the crimes.

In particular, Pacciani and Vanni frequented Salvatore Indovino, a self-styled occultist and fortune teller originally from Catania, at a farmhouse located in the countryside of San Casciano, where, according to local rumours, orgies and rites took place.

[17] The so-called esoteric trail is also linked to the large sums of money that Pacciani came into possession of during the years of the crimes, which gave rise to the idea that the "snack buddies" acted on behalf of personalities who remained unknown.

The checks carried out by the State Police highlighted that Pacciani, before the crimes attributable to the Monster of Florence, was in modest economic conditions and did not inherit assets that could justify the sums of money considered for the most part out of league for a simple farmer like him.

Pacciani, a modest farmer, even had 157 million lire at his disposal (corresponding, in 1996, to 117,069.52 euros in 2018[18]) in cash and interest-bearing postal vouchers, as well as having purchased a car, two houses and completely renovated his home.

Arguments against Pacciani as a murderer hired by mysterious unknown instigators point out that the farmer, in addition to renting an apartment, carried out many odd jobs and was known for his stinginess, as underlined by Giuseppe Alessandri in the book The Legend of Vampa.

Furthermore, the alleged accomplice Lotti was far from rich given that in the 1980s and 1990s he found odd jobs and accommodation only thanks to the help of the town priest, being at all effects a destitute unemployed person.

[19] In 2010 Pier Luigi Vigna, former Florence prosecutor who dealt with the case, declared himself skeptical about the existence of a possible second level of instigators, demonstrating the fact that the investigations following those of the "snacks buddies" have not had any developments .

[20] Based on Lotti's statements regarding a doctor as one of the instigators, Giuttari, the chief prosecutor of Perugia Giuliano Mignini and Gabriella Carlizzi, editor-in-chief of the magazine L'Altra Repubblica, speculated that a pharmacist, Francesco Calamandrei, and a deceased physician from Perugia, Francesco Narducci, had been involved in the secret society ordering Pacciani and the others.

While Mignini claimed the intercepted phone calls made references to Narducci as well, those didn't occur until months after the investigation had been opened and its existence leaked to the public[23][24] The Perugia Public Prosecutor's Office hypothesized that an unknown body was passed off as the deceased doctor at identification, and no post-mortem was carried out when the body was recovered from the lake.

Furthermore, a friend of Narducci, the lawyer Alfredo Brizioli, was also accused of trying to force the medical examiner to draw up a false opinion on the doctor's death.

[25] In 2018 the "esoteric lead", and in particular the direct involvement of Francesco Narducci in the murders of the Monster of Florence resurfaced during the investigation into the disappearance of the Rossella Corazzin in the Belluno area in 1975, as stated in the final draft of the report of the Bicameral Anti-Mafia Parliamentary Commission.

[27] In 2017 Francesco Amicone, a freelance journalist, conducted an investigation on his own that led him to find a connection between the Monster of Florence and the Zodiac Killer cases.

[28][32] During a phone call on 12 September 2017, Bevilacqua implied his responsibility in both the Monster of Florence and the Zodiac Killer case, agreeing to Amicone's request to turn himself in.

[1][28] During the meetings in 2017 Bevilacqua told Amicone he was an undercover CID investigator operating in California, United States at the time of Zodiac's activities in 1969 and 1970, and participated in the CID inquiry on the so-called "Khaki Mafia", which involved SMA William O. Wooldridge, other Army sergeants,[33] and firms from Los Angeles and the San Francisco Bay Area as well as in Reno, Nevada, U.S.[28][34] Following a complaint against Bevilacqua by Amicone dated 1 March 2018, the journalist's first news articles on the "Monster-Zodiac connection" were published by tempi.it and Il Giornale in May 2018.

[34] and that Bevilacqua replaced the pieces of evidence with spent cartridges shot by the gun he would use in the Monster's homicides to link his future crimes to those murders for which he had an alibi.

[28][31] In 1968 Bevilacqua was in Vietnam, but according to Amicone he could have had access to Stefano Mele's trial file where bullets and shell casings of the Signa murder had been improperly stored and switch them to attribute the crime to himself.

[34] In this regard, in 2021 Amicone attached to an addition to the complaint against Bevilacqua a report containing 21 interviews with ballistics experts and the results of a test at the range.

[36] According to Amicone, the report would show that the bullets and casings from the Monster's gun found in the Mele file may not be the same as in 1968, providing a reconstruction of the possible mislead.