It was May 1996, and David Kaczynski, a counselor for troubled youth in upstate New York, sat down to write a letter to his brother Ted. A month earlier, his brother had been shockingly unmasked as the shadowy Unabomber, responsible for a 17-year campaign of bombings that had killed and maimed people across the country.

Ted Kaczynski, a brilliant but mentally troubled mathematician who had retreated years earlier to a remote hovel in Montana, had been arrested based on information from a tipster to the F.B.I., ending one of the longest and most expensive manhunts in American history. He was now in custody and facing what would almost certainly be a lifetime behind bars, if not a death sentence.

The tipster was David.

Sitting in his home in Schenectady, N.Y., David began writing the letter. He used a pencil, knowing he might have to erase before he got it right.

“I could only imagine how much Ted resented me,” he recalled in an interview. Would Ted consider allowing him to visit, he wrote, and try to explain? “I wanted to tell him in person that we morally felt an obligation to stop the violence,” he said.

Ted declined to put David on his visitors list, and when he wrote back, it was to turn the fury of his resentment on his brother.

“You will go to hell because, for you, seeing yourself as you really are will truly be hell,” he wrote.