If it happened outside campus grounds?

"Well, the spark ward in particular goes all the way to the outer Hogsmeade fence on the one side and the far end of the forest on the other, but most everything else's just on the castle. Plus, I mean, most kids go home for the holidays." 

She wonders, actually, whether it would be possible to (Legilimantically?) trace forwards, from a known-good starting point, into the discontinuity... 

"Oh, that would be such fun... Most people's memory isn't nearly linear-episodic enough for that, unfortunately, they'll tend to file memories in metaphorical boxes by type or theme or person or place-- sometimes you see some really neat network graph type setups-- imagine if you could teach somebody to file them in chronological order and then use them as a reality check, Mad-Eye would give you a medal--"

"--no, sorry, irrelevant, psych nerd time later, attempted-murder mystery now." 

"I thought we could try tracing the project."

"How do you mean?" 

"The project was tampered with.  But we know she was legitimately working on it.  So we start from the beginning, from when it was assigned - and work forwards along its timeline, which would be relatively episodic, or at least I suspect - to the discontinuity of someone tampering."

"Ah. Well, she should be medically allowed to talk to you now," (they've now been investigating for quite some time), "so interviewing her does seem like a reasonable strategy. Let's see, what else did you have on the follow-up list..." 

That mess with ?Brutus's Relative's Auror Partner? that she should have the name of somewhere in here is probably not necessarily...

Oh, her name was Redwood. Offensive cursebreaker specialist. Charming, Ravenclaw, turned out to be specialized so hard she was made of glass when it counted. It's probably not related? ... it's bad practice to assume but Cedrella cannot currently see how it would be related. 

Oh hell.  It might be.  She still hasn't figured out a more plausible method for the leak of Septimus's medical information.  Someone who has a family member with a similar condition?  But would they know? Someone who has - no, they couldn't - oh, but if they Confunded Nelya into doing it they wouldn't have had to touch the paper, and mania is like that -

What? 

Was Redwood close enough to Septimus - or simply observant enough to have noticed anyway in passing - in a way that could have left her as the weakest link if someone wanted to find ways in which he was vulnerable?

(The rest of that line of thought should be taken separately.)

"We weren't particularly friends but 'observant' is the sort of thing offensive specialists have in spades, yeah. Never play poker with Potter-- Euphemia, I mean, Monty can't observe a Bludger flying directly at his head." (He says this with great fondness.) "'Specially if he has a potion to look at instead. I'd introduce you but he's kind of insane about--" 

Pause. Staring puzzledly into the middle distance. 

"---hm," he says, and then, very uncertainly, "discontinuity?" 

Cedrella squints at him and then looks quite alarmed. "What the fuck? ... Stop thinking about that immediately," she orders flatly, and then adds to Ophelia, "remind me later." 

Blink blink. "'Kay. Back to the list..." 

Also the Incredibly Suspicious Murder Investigation't is rather concerning in its own right!  Who would benefit from covering that up?

Ah. Well.
You See. 
Uh.

Does he have to explain this one. 

... Fuck, it probably is related, isn't it. And Nelya's safely off in the hospital wing at this point. 

So. 

Cedrella did that one.

Dolohov was angling to get Sept hauled up on negligent homicide charges, see. And to be clear he is innocent of that but he was at the time absolutely not competent to stand trial and explain himself to the bloody Wizengamot without ending up in fact doing several actual murders. This is not their best work on the problem-solving front but in his defense the first couple skirmishes of the war were a real bad time for everyone. 

(The thing with Ted and Septimus seems very obviously like either a staged or real fight about training vis-a-vis How To Survive Duelling Arcturus Black, at least, and while she's going to double-check with them, she doesn't think it's related to the murder attempt.)

Neither Ted nor Sept report any specific episodic memory of the described conversation but they'll both guiltily admit that that is exactly what it sounds like when they're both stressed and pissed off and doing a terrible job choosing diplomatic words and is definitely within plausible normal 'yelling without really thinking about it in a way you don't remember the exact words later just the negative feelings' range.

If she talks to them separately it will become clear the underlying problem is that they disagree on the factual question of whether adding additional duelling ability, and the attendant confidence, will cause Ted to try to fight Arcturus on purpose. 

(why would I do that, that would be Fucking Insane!!) 

(as someone who was certifiably Fucking Insane at that age,) 

(Though perhaps she should have been less quick to rule out...)

(...no.  No, that doesn't make sense at all.  It's not the Blacks.  That matter was surely settled.)

Damn straight it was. 

(But it could be that Narcissa has more information about Septimus than she originally thought she would.  But she's still pretty sure that anyone who could've had information from Narcissa inform their plotting, would have known that Narcissa is at least somewhat invested in her.  And maybe they took that risk, but why?)

Well, given that Ophelia's presence seems to have caused it to not work, it's possible she was not perceived as a potential victim of splash damage and therefore not as a risk factor? Though that itself is suggestive, isn't it, since you'd have to not know how often she's here, right. 

(...) Are broken bones just good clean fun around here?!

Yes. 

(But she doesn't actually think there's someone passing coded messages through discussions of the local Quidditch.  Too easy to notice discrepancies.)

It does seem unlikely. Slytherin typically fields a competent Quidditch team but it's not usually the subtlest members of the species who sign up for it. 

(It is also interesting, but likely completely irrelevant, to note that Augusta Longbottom plays wizard chess against a Malfoy enough for her set to have opinions about theirs.)

Yeah, she and Abraxas have an enchanted scheduling system that gives them a chess date approximately once a month on a random day so that nobody can plan war operations based on it and then they spend the whole time making scripted contentless politeness noises and seething with barely contained murderous fury. Apparently their families have been doing this for like six hundred years and it's because they feel that having to be polite while filled with incandescent hatred builds character. Absolutely insane behavior.  

(...Say.  Madam Cedrella.  Could you invisibly lurk in Ancient Runes and-or Arithmancy classes (as relevant) and feel for if anyone spooks when they see that explosive revealed as an extra-credit reverse engineering project?  She'd do it herself but she's pretty sure she has scheduling conflicts.)

Regrettably that would be unbelievably illegal. 

Oh, you want Runes class. You do need a lot of Arithmancy for Runes but like, in the same way you need calculus to do physics. 

Both less and more than you'd think, then, but certainly not enough to narrow it to those who are noted to have skill in both fields, she figures.

"What the fuck? ... Stop thinking about that immediately," she orders flatly, and then adds to Ophelia, "remind me later."

"Yes ma'am."  Something in the back of her mind has her thinking about the boats, here.  She doesn't know precisely why, but she makes note of that while she is making a note to remind Cedrella of this.

Though that itself is suggestive, isn't it, since you'd have to not know how often she's here, right.

It is.

She's inclined to point suspicious - but most certainly not convinced - fingers at Corban Yaxley, Hendrick Rosier, or one of the Buchanans as an outside guess - of those who are on her social graph, at least.  There could be others.

Reasonable of her, probably. What's her next investigatory step about it going to be? 

Well.  Could any of her suspects have actually done it?  Murder is made of means, motive, and opportunity, after all.

And who, of these people, couldn't have done it?  Who can she prove could not have been able to Obliviate and/or Confund Nelya?