CSI attempt:
Picking out something out of place is as easy as looking for the inert silhouette amongst a background of jade. This strategy quickly pays off, as not far behind the sunken ship you find a length of thick rope leading into the water. It has a series of knots tied at regular intervals towards the end, and judging by the marks in the mud the current has been tugging at it insistently. The Skull River will have to go without this piece of detritus. With the help of the Hammerers you overcome the weight of water and mud and reel it in. It ends abruptly, and after wiping the mud off the end you frown at it. It's heavily frayed and feels stiff to the touch, more so than the fibers in the rest of the rope. So you suspect this was used to hold the explosive in place while the ambushers waited for their prey. You don't see any Aqshy. So no Bright Magic was involved in the explosion, and this confirms the reports that it happened under the waterline. If either or both of those were the case, the Aqshy in the explosion - either directly from a magical component, or drawn in by the moment of detonation - would have been embedded in the rope with such force that it would take days or weeks for it to fade.
It doesn't take long to find what you expected to on the other side of the river - another length of rope tied to a gnarled tree. Pulling that up as well reveals similar fraying and singing to the other end, and an identical lack of Aqshy. So they use the tree as an anchor and hold the rope at the other end. The explosive charge is either weighted or heavy enough to sink on its own, but not all the way to the bottom because it's being held in place. The first ship in the Okral's convoy arrives, and... it explodes. Somehow. Set off by the musket fire? No, it exploded below water. By a pull on the rope? Too easy for it to have been set off by the pull of the river. On impact? That has possibilities. You've read books on Dwarven chemistry that mentioned chemicals that could be ignited by impact or by mixing them with other chemicals or even on contact with water. But while anyone with gold can get their hands on a barrel of gunpowder, those chemicals are significantly more exotic, with much fewer sources...
From this, I think we can safely surmise that the bandits with muskets were solidly a diversion. They were never intended to maximize harm, what they DID do is focus the attention of every dwarf on them so that the actual attackers could slip away.
We might have been able to get better clues, if we had picked the option to search the banks immediately, but by the time we finished the rescue, most of the riverside tracks would be a blur and any lingering Wind traces dispersed.
This they'd expect the dwarves to find in the morning. Material evidence like a rope tied to a tree is not something Rangers would ever be expected to miss.
Over the course of several dives you retrieve several fragments of metal, and when your lungs start to ache you dismiss Branulhune - which disappears with an odd flash of light that it's never displayed before, which is a puzzle for later - and allow the buoyancy of your still-full lungs to bring you back to the surface. When you're unable to find more, you have the boat return to shore, and you spend some time drying out as best you can at the fires that were built for the Dwarves you rescued. When your teeth are no longer chattering, you confer with Belegar and Gotri.
"Steel," Gotri is saying, "not the worst I've seen, but definitely not Dwarven. If I had to guess, I'd say Tilean or Imperial."
"They're all curved back on each other," Belegar observes. "Every one."
"Hoops," you say, "around an exploding barrel."
Gotri turns the fragment over in his hands. "That would cause these stress patterns," he concedes.
"Barrel full of gunpowder - or something else - hanging from a rope, one end of the rope tied to a tree, the other held by the ambushers. It's heavy enough to sink and it's held below the waterline, either to avoid the monitor's armour or to prevent it being seen or both. If they had a particular target in mind they could give it enough slack for the barrel to sink to the bottom and allow non-targets to pass by unharmed. When this monitor arrives, it hits the barrel and it explodes. Don't know the exact mechanism."
"I could think of a few ways to do it," Gotri says after a moment, "but it would require very careful manufacturing, and not minding terribly much if it explodes prematurely."
"It could also be done with magic, and I suspect chemistry. But even if you combine all three possible triggers, that narrows it down a lot more than just a barrel of gunpowder." You turn the steel over and over in your hands. "I might be able to find out more back in Karak Eight Peaks. And if Barak Varr can find the slugs shot at the ship, I could get something out of that too."
This is more in doubt, but given the way the attack was carried out, they'd expect the metal debris to be recovered eventually.
It SLIGHTLY points against Chaos Dwarf work - their pride would make using lower quality steel a tough sell. But it doesn't point TO anything - Imperial origin barrels of
adequatefine(dwarves said its not the worst, so probably pretty decent) quality could be purchased anywhere, whether firsthand or whether they just bought or stole a used barrel for this..
Also we didn't recover anything that seems like a detonator element, but its unsurprising, those would be more delicate bits and likely either buried or washed away by now...as the attackers probably expected.
You'd like to bask in that moment, but like two Kings and more Engineers than you can count, all your attention is fixed on the hole that sunk the ship.
"Dead center of the bottom corridor," says the Grandmaster of Barak Varr's Slotchokri, the Riverine Shipwrights, speaking loudly to be heard over the rushing of the nearby river. "And the blast radius was large enough to partially open up the corridor above and the compartment below. They couldn't have found a more crippling shot if they tried."
"We see this sometimes, from-" a moment of hesitation from the Grandmaster of Barak Varr's Skarrenokri, the Oceanic Shipwrights. You turn to him, and find his eyes on you.
"She knows," Belegar grunts.
"The Frurndar have weapons that strike below the waterline. We've begun armouring against it, but..." He shrugs. "You can only put so much weight on a ship. More steel below the waterline means less above it. The Slotchokri were thought to never need to protect against more than an errant boulder."
The accuracy of the hit is uncanny for a barrel on a rope.
This is professional work, somebody was actively adjusting the barrel's placement likely minutes before impact, if you wanted to just sink SOMETHING you'd string several barrels and make sure the ship would hit at least one. A perfect dead center hit? You can't do that even if you knew the route - ships don't travel dead center of the river, they'd meander randomly in small ways.
Somebody with the ability to triangulate and adjust the barrel positioning on the go.
"A barrel of an explosive substance was held in place by rope," Gotri says. "One side tied to a tree, the other held. The ship hits the barrel, the barrel explodes. Possibly some sort of impact trigger mechanism."
"Gunpowder?" One Engineer says.
"Must have been a hell of a lot of it for a hole that big," says another.
"I'll be able to say how big the barrel was as soon as I get the metal back to my workshop," you say. "From there you should be able to determine whether it was gunpowder or something..." Your voice trails off as you stare at the jagged edge of the hole.
"Something what?" says one of the Engineers, but you're already halfway up a ladder against the side of the hole. The last trickles of bloody water have just finished emerging from the frontal compartment, and without that steady stream of Shyish you think you can see a trace of something, something that fire and water haven't quite washed away. Embedded in the steel here and there are what you'd call splinters if they weren't so pulverized. Magically speaking, many types of live wood are very mutable and can absorb just about any magical energy that it finds itself surrounded by, for better or worse. Dead wood retains these properties to an extent, so prolonged exposure can leave a mark. You wouldn't be surprised to see Ghyran from the river or Shyish from the dead Dwarves in the pulp, but you're not seeing that. Lurking inside the wood pulp is the very faintest touch of Dhar.
"Traces of Dark Magic," you say. "Not from the trigger mechanism, a Dhar trigger wouldn't have lasted long enough for the wood to absorb it. It's from the explosive substance. And I only know of one explosive substance that would radiate Dhar."
"Skaven blackpowder," Belegar says, and you nod. The atmosphere wasn't exactly jovial before, but it darkens even further as hands tighten on weapons.
The yield explained. Skaven blackpowder does have the necessary yield, in a quantity that could be carried by one person. This is also something that the dwarves couldn't be expected to discover. They don't have the means to detect tiny traces of Dhar.
Its not proof positive of Skaven, certainly humans could use Skaven powder, but Skaven would have the ability to set this up, and sparking conflict between humans and dwarves are to their advantage.
As far as Grudges go this is good early findings - barring other discoveries, it'd be yet another grudge written against the Underempire.
...and somebody should go check on the pestilence hitting the human workers on the canal.
Is it Skaven taking advantage of the conflict to get their own licks in, or is it Skaven egging on the conflict?
All told, this event should be a nice validation of Belegar's choices to the Karaz Ankor.
Dwarf lives saved
and a Grudge which might have gone mis-aimed if not for having a master wizard on hand to investigate. All the nonmagical material evidence just points to "Human faction did this", and you could excuse a lot of the yield with "well, magic?"