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Ed Andrieski, Associated Press
El Paso County Sheriff's Lt. Jeff Kramer leaves a news conference near the home of Department of Corrections Director Tom Clements in Monument, Colo., on Wednesday, March 20, 2013. Clements was shot and killed at his home on Tuesday night.

DENVER — Police investigating the shooting death of Colorado's top prisons official appealed to the public for help as they checked surveillance video and pursued numerous possibilities Thursday, including a Craigslist ad the victim apparently posted this week to sell a bicycle.

Tom Clements, 58, was killed as he answered the door to his home Tuesday night in Monument, a town of rolling hills and alpine trees north of Colorado Springs. His death stunned law enforcement colleagues in Colorado and Missouri, where he spent most of his career as a highly respected corrections official.

El Paso County sheriff's investigators were looking for a late-model car, possibly a Lincoln or a Cadillac, that a neighbor spotted near Clements' home around the time of the shooting. Lt. Jeff Kramer refused to say what other clues may have been found after Clements' neighborhood was canvassed by officers.

Denver's KMGH-TV reported Thursday that Clements may have put a bicycle up for sale for $1,200 on Craigslist. Kramer told the station, "I can't speak to the efforts behind this tip, or the level we are giving it."

In recent weeks, Clements had requested chemicals to plan for the execution of a convict on Colorado's death row and denied a Saudi national's request to serve out the remainder of a sentence in his home country. Officials refused to confirm or deny whether they were looking at those actions as possible motives.

Clements came to Colorado in 2011 after working three decades in the Missouri prison system. Missouri Department of Corrections spokeswoman Mandi Steele said Thursday the department was ready to help in the probe if asked.

"Tom regularly commented that corrections is inherently a dangerous business, and that's all that I'll say," said Alison Morgan, a Colorado corrections spokeswoman who worked closely with Clements.

Officials in positions like Clements' get a deluge of threats, according to people who monitor their safety. But it can be hard sorting out which ones could lead to violence. A U.S. Department of Justice study found that federal prosecutors and judges received 5,250 threats between 2003 and 2008, but there were only three attacks during that time period.

The last public official killed in Colorado in the past 10 years was Sean May, a prosecutor in suburban Denver. An assailant killed May as he arrived home from work. Investigators examined May's court cases, but the case remains unsolved.

"We were looking for anybody who had a grudge against May," Jackson said.

Steven Swensen, a former U.S. marshal who runs a company that provides security advice for court personnel, said one problem is that if someone really wants to harm a person, they usually don't send a warning.

"The person who makes a threat isn't the most likely to carry out on a threat," he said.

Glenn McGovern, a senior investigator with the Santa Clara County district attorney's office in California, tracks attacks on judges, prosecutors and senior law enforcement officials worldwide. He tabulated 133 such incidents in the U.S. since 1950.

"When I was looking at these attacks, very few gave any kind of threats," he said. "When it comes up, it's out of left field."

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Mike McLelland is district attorney in Kaufman County, Texas, where one of his prosecutors was gunned down in January walking to the office through the courthouse parking lot.

McLelland said the attack was a "well-executed assassination" and that investigators have had to comb through every case the lawyer, Mark Hasse, handled. McClelland is still baffled at what might have sparked the slaying.

"Nobody is excluded and everybody is included," he said of investigations like these. "They're knocking over every rock they can."

Associated Press writers Colleen Slevin in Denver and Jordan Shapiro in Jefferson City, Mo., contributed to this report.