'Unabomber' Ted Kaczynski Rails Against U.S. in Never Before Seen Letter

Unabomber Ted Kaczynski was more than a domestic terrorist mailing package bombs around America -- he was also a staunch proponent of the First Amendment to the Constitution, at least according to the madman himself.

TMZ has obtained a never-before-seen letter he wrote while sitting in federal prison, furious the US government had seized his infamous trove of personal items from his cabin and planned to auction them off to pay restitution to his victims.

In the letter, dated April 3, 2009, the terrorist railed against the system he claimed had wronged him in his multi-year legal battle with the feds over his possessions.

He complained the government had confiscated "every copy of everything I've ever written," and, worse, the court of appeals had already greenlit the auction ... prompting him to reluctantly admit, "I've lost."

But Kaczynski also warned his case should be of "grave concern" to all Americans because it showed the federal courts totally ignored the First Amendment ... in other words, "freedom of expression," as he put it in his letter.

He went on to say something about the court's decision being politically motivated ... but didn't further explain that theory.

The convicted terrorist wrote and sent the letter to a journalist with whom he was contemplating doing an interview -- about his belongings and the First Amendment -- but he ultimately decided against discussing it any further.

Two years later, the auction took place and, when it was all over, Kaczynski's personal property sold for $190,000, which went to his victims and their family members. Among the items sold were his personal journals, his handwritten "manifesto" and his hooded sweatshirt and sunglasses, as seen in his iconic police sketch.

As you know ... Kaczynski died by suicide inside his prison cell at FMC Butner in North Carolina last Saturday. TMZ broke the story ... 911 audio revealed first responders were unable to resuscitate him after he hanged himself.

Before his death, Kaczynski was serving 4 life sentences after pleading guilty in 1998 to killing 3 people and maiming 28 others during his nearly 20-year mail bomb campaign.