once all the characters had been accounted for and Millenium has been bought into by Vanger's company I had kept going back to Martin and why he wasn't mentioned more in reference to Harriet's disappearance, that and the connections pointing to Cecelia and Anita tends to be incredibly strong.
So I kept having a running theory that the two sets of murders were separate but related through Harriet, that Martin was involved somehow in Harriet's disappearance and had an accomplice. The reason being was that he was immediately thrown out as a suspect because he was in Upsalla.
plus Cecelia was either a witness or an accessory. I can't figure out whether Harriet is alive or dead.