If you were surprised to read his wife is by Rossini's side for his investigations, know that he never successfully managed to have sex with Liz, which was noted in her autopsy. This is apparently enough to make Mrs Rossini forgive her husband's attempted indiscretions. Just as well Liz retained some of her virtue, or else our intrepid hero's reputation-and marriage-would've been ruined by her slutty, slutty ways!
If you're surprised that Rossini (and forgiving wife) choose, and are allowed, to get stuck into the official investigation despite being teachers with absolutely no formal connection with the police, know that I am too. Especially given he showed no inclination towards amateur sleuthing when he was actually under suspicion for the schoolgirls' deaths. He's driven by something, though, to bring the mysterious 'bearded priest' to justice. If I had to guess what's at the heart of his motivations, I'd suggest that it's the same two things that lie at the heart of the film in general-a reverence for female purity and a reverence for tits.
Tits-wise, Rossini is all about bending Liz to his sexual will, to the extent that he cultivates the love nest, replete with posters of topless chicks stuck about the bed, to try and seal the deal. Likewise, the film contains two shower scenes, ostensibly to set up one of Rossini's fellow teachers as a red herring, but really as an excuse to revel in as much boobs and bush as possible. The second such scene contains an extended shot which cuts the girls' heads off, and literally renders them faceless flesh objects. The murder scenes, too, all contain female nudity.
This is where the tits and purity intersect. These gritty, unpleasant scenes don't overly linger on the victims' bodies (which are nonetheless on full display); instead the intention seems to be to shock and appal the viewer. There seems to be an idea at play regarding the destruction of youthful innocence (even if it's unintentional, it's there). The schoolgirls, while never really portrayed as paragons of virtue, are young and beautiful. The film knows this, and appreciates it as much as Rossini, with his lusting after his students, does. And, when this youth and beauty are destroyed, the person responsible must be brought to justice. This seems to be what drives Rossini, even though he himself was engaged in a concerted campaign of attempted corruption (albeit corruption of a less extreme type). The fact that he's only motivated to investigate after Liz's murder, coincidentally the murder that proves his innocence, backs this up-he didn't have any inclination to do anything to clear his name himself.
My description of sex as 'corruption' just there was very deliberate. Given that this is an Italian film, and an Italian film from almost 50 years ago, it's not exactly progressive in its thinking. The men aren't judged for being pussy hounds, whereas the women bear all the consequences. (Note that in a 2006 interview, producer Fulcio Lucisano stated that the film is essentially conservative and anti-abortion.) Rossini does lose his job due to his indiscretions, but you don't feel that'll overly inconvenience him once his quest to avenge his virgin lover is over. The teacher who looks but doesn't touch is superficially painted as a sweaty pervert by the film, but isn't the film just as sweaty and just as pervy? In a literal sense, no, it's not, but in a metaphorical sense, it sure is.
It is a beautiful film in many ways, though. Ennio Morricone's soundtrack is sublime, among his very best work. Joe d'Amato (/Aristide Massaccessi, a man whose twin personae perfectly encapsulate the film's straddling of respectability and base exploitation) does a great job with the cinematography, and Massimo Dallamano is an assured director. It's a long film, though, and not exactly thrilling. Dialogue abounds, and, apart from one nighttime murder, which briefly hits lightning-and-shadows gothic heights, there's very little suspense. There's no appetite for showing beautiful things in peril, only in seeing them elevated (and bared) or destroyed.
The investigative plot proceeds at a fairly stately pace, and yet there are many strands which feel slightly incomplete, and not fully played out. This was most likely an editing decision, and necessary to keep the running time under two hours. As long as you stay alert, this slightly fractured approach to plot works quite well. In particular, scenes in which the police discover evidence are in short supply; instead we jump straight to said evidence being produced in front of suspects. This is an efficient way of streamlining, and actually can subtly cast suspicion upon the police as we momentarily wonder if they've manufactured evidence.
Said police do play a larger role than in most gialli, no doubt partly due to the German financing, and the film's being marketed there as an Edgar Wallace kimi (the edited, nudity-lite version may well have resembled a krimi, but the plot had nothing to do with any Wallace story). The krimi link may also account for the London setting, as a large amount of those films were set in Wallace's native land. It certainly wasn't set in the UK for reasons of verisimilitude, as the incident which sparks the plot into life wouldn't have occurred in the early 70s, thanks to then-recent legislative changes...
The exact nature of this incident is one of the twin mysteries at the heart of the film, and is the answer to the titular question. The other is the standard 'who is the killer?' mystery, which breaks from giallo tradition by almost being solvable through rational Holmesian logic. If you pay close attention during certain scenes, and question the provenance of certain information possessed by some of the characters, you should be able to crack the case. As usual, you'll have access to more info than the characters within the film, which might explain the bizarro left-field reasoning through which Rossini lands on his main suspect (reasoning which is flawed anyway, due to either a mistake in the subtitles or scripting; just pay attention to what the photographer says on the boat, and compare that to Rossini's later logic).
The killer's reveal, which is possibly a belated attempt to generate suspense, doesn't really succeed, as it's dragged out interminably over close to ten minutes before a soundtrack sting accompanies the final, absolute, incontrovertible proof of the killer's identity. An identity which Rossini, first with his wife and then with wife and police, has spent the previous seven or eights minutes assiduously trying to establish. Karin Baal, in her hilariously forthright interview on the Arrow Blu Ray, says that the film is overlong and "not a thriller."** This is probably a fair summation, and it's possible that Dallamano wasn't even trying to make an exciting thriller, and was instead attempting something more beautiful, more fragile and more profound. What he came up with was sometimes beautiful, occasionally fragile, and profoundly uneasy with both itself and its women.
If it seems like I'm overly-fixating on the treatment and portrayal of women in this film, get ready for the next two instalments of Dallamano's 'Schoolgirls in Peril' trilogy...
*Or, if you're Jay Z, What Has Solange Done to You? That's a different Solange, though, so isn't relevant. Giallo fans should ignore the previous two sentences, and probably this one too. In fact, just ignore this footnote altogether.
**Her descriptions of Fabio Testi's approach to dialogue-learning certainly make for an interesting contrast to his own self-aggrandising in another interview on the disc, and will make it very difficult to rewatch the film without wondering where he was hiding his dialogue coach in any given scene.