Complex sentence: The Illinois State Police was the first major law enforcement agency to carry a semi-automatic pistol, starting in 1968 with the Smith & Wesson Model 39, which it carried until 1981, transitioning then to the 2nd generation Smith & Wesson Model 439.
Split into simpler sentences: The Illinois State Police was the first major law enforcement agency to carry a semi automatic pistol starting in 1968 with the Smith & Wesson Model 39. ISP carried the Smith & Wesson Model 39 till 1981 transitioning to the 2nd generation Smith & Wesson Model 439.