DENVER — As Colorado’s governor signed a hard-won package of gun control measures on Wednesday, officials across the state were reeling from the seemingly inexplicable shooting death of the state’s prisons chief, who was gunned down at the front door of his home.

The killing of Tom Clements, a man described by friends and colleagues as a dedicated and thoughtful public servant, left state officials shaken and grasping for answers on Wednesday. State troopers increased security around the State Capitol, and some state workers said Mr. Clements’s death had put them on edge.

The state police said they had known of no specific threats against Mr. Clements before 8:30 p.m. Tuesday, when someone approached his house in the pine-fringed hills of the town of Monument, near Colorado Springs, and shot him as he answered the door. Into Wednesday night, investigators were still searching for any trace of his killer, but said they had no suspects or motive.