The evidence is not conclusive -- although I doubt one of the mothers in the church would have strangled her own baby -- but it is very, very suggestive that these people were murdered by the French. Esdaile THE PENINSULAR WAR (P. 330) states that 200 men, women and children lost their life in this atrocity but his source for this figure is not clear from his notes.
It would seem to me that the atrocity at Porto de Mos would be well known in Portugal. Perhaps some of our Portuguese readers could check whether there is more information available in the Portuguese records.
dg