Yes, it is highly illegal to harm or assault retired federal law enforcement officers on account of their past official duties. [1]

Under federal law (18 U.S.C. § 111), it is a specific federal crime to forcibly assault, resist, oppose, impede, intimidate, or interfere with any person who formerly served as a federal officer or employee due to the performance of their official duties during their term of service. [1]

The penalties for these acts are severe and are structured based on the severity of the offense: [1, 2]

• Simple Assault: Fines and/or up to 1 year in federal prison.
• Assault Involving Physical Contact or Intent to Commit Another Felony: Fines and/or up to 8 years in prison.
• Assault Involving a Deadly Weapon or Inflicting Bodily Injury: Fines and/or up to 20 years in federal prison (18 U.S.C. § 111). [1, 2]

For more details on the federal protections afforded to former and current personnel, you can review the United States Department of Justice Criminal Resource Manual. [1]