JERUSALEM (JTA) — An Israeli right-wing activist in prison for illegal political activity was arrested for sending a threatening letter to Prime Minister Benjamin Netanyahu.

The prisoner, who was not identified in Israeli media reports, is in a central Israel prison and is 22 years old. He has been accused of involvement in “price tag” attacks and issuing threats to left-wing leaders.

In the letter, the prisoner expressed his opposition to Netanyahu’s decision to release Palestinian prisoners in order to jump-start peace talks. Israel recently released the first group of 26 prisoners; it is scheduled to set free a total of 104.

“How are you releasing murderous terrorists? We’ll hurt you and the government like Rabin was hurt,” the letter to Netanyahu said, referring to assassinated Prime Minister Yitzhak Rabin.

He was arrested in prison and found to have at least one knife in his possession.