﴿ أَلَآ إِنَّهُمْ هُمُ ٱلْمُفْسِدُونَ وَلَٰكِن لَّا يَشْعُرُونَ ﴾

“Oh, verily, it is they, they who are spreading corruption but they perceive it not?”

Truly a-lā ‘truly’ is for alerting intended emphatically they are the agents of corruption but they perceive this not.