MONUMENT, Colorado: Colorado's top state prison official was shot and killed when he answered the front door of his house, setting off a hunt for the shooter and raising questions about whether the attack had anything to do with his job.

Tom Clements, 58, was shot around 8:30 p.m. Tuesday in Monument, north of Colorado Springs, and a witness reported a person driving away in a dark-colored "boxy" car that had its engine running at the time of the shooting, authorities said.

Investigators were exploring all possibilities, including that the shooting could have been related to Clements' job as executive director of the Colorado Department of Corrections, which he took after years working in Missouri corrections.

The killing stunned officials in both states.

At a news conference, Colorado Gov. John Hickenlooper was red-eyed and somber, speaking haltingly as he said he didn't think the killing was part of any larger attack against his cabinet, members of which stood behind him, several of them crying. Others dabbed their eyes.

"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," said Hickenlooper, who planned to go to Monument to meet with Clements' family after signing gun-control bills.

While the motive of the killing wasn't immediately clear, similar attacks on officials have been on the rise in the U.S., 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 as many in the past three years as the entire prior decade.

The attacks are often motivated by revenge, said McGovern, who has documented more than 133 attacks, including 41 homicides, against judges, prosecutors and other justice and police officials since 1950.

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. He cited al-Turki's refusal to undergo sex offender treatment.

Homaidan al-Turki, a well-known member of Denver's Muslim community, was convicted in state court in 2006 of unlawful sexual contact by use of force, theft and extortion and sentenced to 28 years to life in prison. Prosecutors said he kept a housekeeper a virtual slave for four years and sexually assaulted her. A judge reduced the sentence to eight years to life.

Al-Turki insisted the case was politically motivated. He owned a company that some years ago sold CDs of sermons recorded by Anwar al-Awlaki, killed in a drone strike in Yemen in 2011.

Al-Turki's conviction angered Saudi officials and prompted the U.S. State Department to send Colorado Attorney General John Suthers to Saudi Arabia to meet with King Abdullah, Crown Prince Sultan and al-Turki's family.

After Clements' shooting, someone with the State Department called the Colorado Corrections Department. Prisons spokeswoman Alison Morgan said she had no details on the call other than to say it wasn't connected to the shooting investigation and may have been a simple courtesy.

"They called us because we have a cooperative international program with them," she said.

Attorney Henry Solano, one of al-Turki's attorneys, said he has not been contacted by investigators. He refused to comment on the shooting.

Hickenlooper appointed Clements to the post in 2011. He replaced Ari Zavaras, a former Denver police chief who led the department under two governors.

Since October 2011, his wife, Lisa Clements, has been the director of a state office that oversees the state's mental health institutes in Fort Logan and Pueblo, as well as community mental health and substance abuse centers.

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