By NICHOLAS RICCARDI and P. SOLOMON BANDA

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.

Tom Clements

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.

"It's often taking place away from the office, which makes sense, because everyone's hardening up their facilities," he said, adding that he advises prosecutors to constantly assess the safety of their residences.

On Jan. 31, Texas prosecutor Mark Hasse was gunned down as he left his car in the parking lot to the county courthouse. McGovern also counts the rampage by an ex-Los Angeles police officer who killed the daughter of a retired city police officer as part of a plot to avenge his firing.

In Colorado, a prosecutor was fatally shot in 2008 as he returned to his Denver home. In 2001, federal prosecutor Thomas Wales was fatally shot by a rifleman while he worked on a computer at night in his Seattle residence. Both cases remain unsolved.

Attacks on legal officials are still extremely rare, said Scott Burns of the National District Attorneys Association, which counts 11 prosecutors as having been slain in the last 50 years. But he acknowledged that legal officials are vulnerable outside of protected offices and courthouses.

"If someone wants to truly harm or kill them, it's very difficult, frankly. There's not a lot we can do," he said.

Mike McLelland, the district attorney in rural Kaufman county east of Dallas, is a 23-year military veteran. Since his prosecutor, Hasse, was killed on his way into the office, McLelland has warned his staff to be vigilant about their surroundings and possible danger.

"The people in my line of work are going to have to get a lot better at it, because they're going to need it more in the future," McLelland said, adding that he carries a gun everywhere he goes.

Colorado Corrections spokeswoman Adrienne Jacobson would not comment on whether Clements had security at his home. Security was stepped up for other state officials, including Gov. John Hickenlooper, who was ashen-faced as he addressed reporters at the capitol Wednesday.

"Tom Clements dedicated his life to being a public servant, to making our state a better place and he is going to be deeply, deeply missed," Hickenlooper said. In response to a question, he said he believed the rest of his cabinet was safe.

Clements came to Colorado in 2011 after working three decades in the Missouri prison system. He began a review of Colorado's solitary confinement system. He reduced the number of prisoners being held in solitary and closed a new prison built specifically to hold such prisoners — Colorado State Penitentiary II.

He lived in a wooded neighborhood of large, two-story houses on expansive 2-acre lots dotted with evergreen trees in an area known as the Black Forest. Long driveways connect the homes to narrow, winding roads that thread the hills. After word of the shooting spread Tuesday, residents slept with shotguns at the ready, fearful the shooter would return.

It would have been simple to find Clements' house. It took two clicks to get his correct street address through a publicly available Internet locator service Wednesday morning. The listing also included his previous home address in Missouri.

McGovern said he tells his prosecutors to assume that any possible assailants can find their home addresses online and to check for areas they may be especially vulnerable such as neighboring alleys and poorly lit porches.

There is no central database of attacks on legal officials and senior law enforcement executives like Clements.

McGovern has documented 133 of them in the U.S. since 1950 by searching through news accounts and court cases. The total includes 41 killings of judges, prosecutors and other justice and police officials. The assaults usually come with little warning, he said.

Steven K. Swensen, a former U.S. marshal who runs a business consulting on security for court officials, said that attacks on legal staff used to occur in courtrooms. As security has been expanded to protect those rooms, then courthouses, the attacks have spilled out further and further.

"Now we're having more violence off-site, in judge's houses, on their way to and from work," Swensen said.

While Clements generally kept a low profile, his killing comes a week after he denied a request by a Saudi national to serve out the remainder of a Colorado prison sentence in Saudi Arabia.

Clements also recently requested chemicals to execute Nathan Dunlap, who was convicted of killing four people during a 1993 shooting rampage at a Chuck E. Cheese restaurant and is scheduled to become the second person executed in Colorado since the U.S. Supreme Court reinstated the death penalty in 1976.

Clements' father-in-law, Carroll Smith, told The Denver Post that Clements opposed the death penalty.

Earlier this week, Clements spoke to legislators about the need for more security staff in the department's food service areas. Last year, a kitchen worker at a state prison was killed and another was injured in an assault involving an inmate.

Clements is at least the second state prisons chief killed in office. Michael Francke, director of the Oregon corrections department, was stabbed to death outside his office in 1989 in what prosecutors described as a bungled car burglary. A former Oregon prison inmate was found guilty of aggravated murder in 1991 and sentenced to life in prison.

Hickenlooper ordered flags lowered to half-staff at public buildings until the day after Clements' funeral.

Contributing to this report were Associated Press writers Thomas Peipert in Monument, Colo.; Steven K. Paulson, Dan Elliott, Nicholas Riccardi, Alexandra Tilsley and Colleen Slevin in Denver; and Maria Sudekum in Kansas City, Mo.