Couldn’t be. There were simply not enough Crusaders even to set up a seige. There is no possible way for them to have sealed off the city and conducted a general massacre. Maybe all that remained to fight would have been put to the sword, but even that seems unlikely. The general rule was to keep captives for slaves and ransom/
This I know for a fact is not true. I have read a great deal about the Hundred Years War and the rule was kill all but the knights who might pay ransom. Since I posted to you I have checked 3 on line encyclopedia sources about the First Crusade and they all agree that there was a massacre of some size and that it was common practice in that day.