I was climbing in another sector at Havukallio when the accident took place and helped the emergency services. I didn't know the people involved and perhaps they will be able to comment about the accident itself. I abseiled down the route to clean in and inspect the cause and will offer a factual account of my observations. I hope that the person involved makes a speedy recovery and thank the emergency services for their quick arrival.
The person involved was climbing one of the sports routes, he didn't reach the anchor and lowered off from the last bolt. There was an in-situ maillon (mailonen) there, which he lowered from. He removed the quickdraw under that and then the maillon broke, which dropped him to the ground.
The maillon involved was a 5mm Maillon rapide. You can see an image of it here http://desert-climbing.blogspot.com/
The working load of this maillon should have been 250 kg and the breaking limit should have been 1250 kg. But, and this is a big issue I would be fairly sure that the gate was open at the time of failure. I say this because the screw threads are perfectly intact and not in the slightest part damaged as though they had been ripped apart. No "gate open" strength is given for such maillons and according to one source who worked in the test lab for Lyon (the UK importer of maillons), "Maillons don't have a gate open strength marked - as is already mentioned, they must be done up. Cavers, who use maillons a lot, know that just bodyweight can deform them if not screwed fully closed. In fact, the threads and barrel are lined up by hand during manufacture, by bending the frame."
It could be that the action of the rope running through the maillon opened the gate. This is easier to imagine if many people had lowered off the same maillon.
Other important points to remember are that when someone is lowered the force generated on the anchor is twice that if they were abseiling due to the pulley effect. This force is further increased if someone swings about e.g. to collect quickdraws. As mentioned a 5mm maillon has a working load of 250kg http://www.peguet.fr/gb/produits/grande_ouverture.html
If you use a larger diameter maillon you increase this margin dramatically.
I would say to others - to avoid this happening again -
If you have to use a maillon then make absolutely sure that the gate is firmly closed and that the rope can not open it.
On Finnish routes I suggest lowering off normal gear e.g. a screwgate or quickdraw, walking to the top of the cliff and collecting the rest of the gear by abseil.
If you remove the runner directly below the one that you are lowering from, you leave yourself no safety margin in the case of that runner failing.
The larger maillon you have, the bigger the safety margin is in terms of strength and in terms of size. The latter may be important as the rope would be likely run clear of the gate, meaning that it would be incredibly unlikely that the rope would open the gate.
I'm sure that the use of such maillons is widespread in Finland. There was a 5mm maillon attached to one of the two anchor bolts. This was firmly shut and I couldn't move the gate at all. It is common to glue in-situ maillons shut and this could be what has happened to the top one. I hope that people can learn from this and that this never happens again.