Daniel 9:6

“Nor have we listened to your servants the prophets, who spoke in your name to our kings, our princes, and our fathers, and to all the people of the land.”

They had added to their sins in that they had refused to listen to the words of the true prophets, who had spoken in YHWH's name. All were involved in this, from the king downwards. Compare Jeremiah 7:25; Jeremiah 25:4; Jeremiah 26:5; Jeremiah 29:19; Jeremiah 44:17; Jeremiah 44:21; Nehemiah 9:32; Nehemiah 9:34; Ezra 9:7. The verses in Jeremiah demonstrate where Daniel obtained his ideas from, but he had distant memories of having seen it for himself. The references in Nehemiah and Ezra are more formal indicating that they come later than Daniel.