MONUMENT, Colo. – The fatal shooting of Colorado's top prisons official when he answered the front door at his house highlights a troubling reality for the nation's judges, prosecutors and other legal officials: At a time when attacks on them are rising, it's difficult for them to remain secure, even when they are off duty.

Investigators do not yet know why Tom Clements, 58, was shot around 8:30 p.m. Tuesday at his home just north of Colorado Springs. They could not rule out any possibilities, including that it was a random shooting or that it was an attack related to Clements' job, authorities said.

While small in numbers, similar attacks on officials have been increasing in the U.S. in recent years, said Glenn McGovern, an investigator with the Santa Clara County District Attorney's office in California who tracks such incidents worldwide. He said there have been roughly as many in the past three years — at least 35 — as the entire prior decade. Revenge is usually the motive, he added.