witness to the crime
They took countless precautions, accounted for every possible variation. Everything was meticulously plotted down to the last detail and after that they even waited until the weather was absolutely perfect. Clear and bright, with no wind to carry screams and convenient rain forecasted for the evening to wash any remaining evidence away.
Everything went according to plan, so smoothly that there were jokes made about the ease of the thing, that perhaps they over-planned. There were no mistakes, no missteps. The crime was carried out so quickly that there was not a single scream left hanging in the air by the lack of wind.
Of course, they also thought there were no witnesses, but the pigeon saw everything. No one saw the pigeon, as the pigeon was not an accounted-for variation in the weeks and months of planning.
Pigeons have no loyalties, and they keep no secrets. Before the rain came every bird in the city knew what had happened, their own plans already formulating.