How were the letters of St. Ignatius of Antioch changed during the fourth century?

During the 4th century, the original letters of St. Ignatius of Antioch underwent significant corruption. An interpolator made heavy insertions into the genuine texts, altering their original content.1

Furthermore, the collection was expanded through the addition of six forged letters written under Ignatius’s name. This enlarged and corrupted collection remained widely known throughout the Middle Ages until 17th-century scholarship eventually restored the genuine letters to their original form.12