Vielleicht hätten die Morde sogar verhindert werden können. Denn der Verfassungsschutz kannte die Täter und weitere Personen aus ihrem Umfeld seit den 1990er Jahren, hatte aber irgendwann ihre Spur verloren. Is the English equivalent of denn here "because"? It seems odd: from denn (because"?) to the end isn't a sentence. If the idea were written in English, there could be a colon after können, and the next words would be "the Office for ......... ." ("Perhaps the murders could even have been prevented: the OFPOTC knew those who were responsible and others they mixed with, but at some point lost the trail.") Anyway, as it stands, from denn to the end isn't a sentence.