What is the main conflict in Death of an Expert Witness by P. D. James?

Death of an Expert Witness by P. D. James takes place in Chevisham, England, sometime in the late twentieth century. Dr. Edwin Lorrimer (the "expert witness") is murdered in his workspace at Hoggatt's Lab, a forensic crime lab. Further on in the book, a middle-aged woman, Stella Mawson, is also murdered by the same man, Dr. Kerrison. Commander Adam Dalgliesh, a Scotland Yard Detective, using the few clues available, must eliminate numerous persons with the motive to kill Lorrimer to finally bag his man.