Colorado prison chief killing: Saudi national's case considered NEW: A Saudi's request to finish his sentence in Saudi Arabia was turned down by Clements Tom Clements, 58, was gunned down Tuesday night after answering his door at home Investigators say a man was seen driving away in a black, boxy vehicle El Paso County, Colorado (CNN) -- While the investigation into the slaying of Colorado's prison chief is "wide open," a top law enforcement official told CNN that one case involving a Saudi national is being considered. When asked if any cases have risen to the top in the search for who killed Tom Clements, El Paso County Undersheriff Paula Presley brought up a case involving Homaidan al-Turki. There has been, you know, one case in specific that has been mentioned, certainly that the media has mentioned, with al-Turki, who was denied transfer back to Saudi Arabia. And certainly that has been in the media in the last 24 hours," Presley said, referring to reports in both the Denver Post and CNN affiliate the Denver Channel. A week ago Clements denied al-Turki's request to serve out the remainder of his Colorado prison sentence in Saudi Arabia, records show. Al-Turki was convicted in state court in 2006 of 12 counts of sexual contact, theft, extortion and false imprisonment and sentenced to 28 years to life in prison, according to court documents. Clements refused al-Turki's transfer request because, records show, al-Turki refused to go through sex-offender treatment. In a letter dated March 11, Clements wrote to al-Turki: "To date you have reportedly declined due to religious reasons/conflicts with your Islamic faith." CNN has confirmed that Colorado's Attorney General John Suthers traveled to Saudi Arabia to meet with officials there to discuss al-Turki's incarceration. CNN is making calls to al-Turki's lawyer. Read the full letter "We are assessing that information, determining whether that might've been, there may have been some motivation or legitimate threat," Presley told CNN, speaking in general terms about any threat Clements might have received. Regarding al-Turki, she said, "But, again, we have not identified that specifically as a threat." Meanwhile security was beefed up at the governor's office and other locations statewide Thursday. Clements was shot dead Tuesday night after opening the door at his home in Monument, about 50 miles south of Denver "We are looking at all potential tips, leads, threats that Mr. Clements may have had from anybody in that prison system," Presley said on CNN on Thursday morning. The investigation is wide open at this point. Read: Searching for a car in prison chief's killing Asked whether the 58-year-old's killing might have been a professional hit, Presley replied, "We don't have any specific information that would lead us to that." Clements was appointed to his job by Colorado Gov. John Hickenlooper, who signed landmark gun control bills Wednesday. Hickenlooper said that Clements was "supportive but not particularly active" in the measures, and there is no indication now whether the new laws and his death are related. Investigators are asking any witnesses to share information about a black, boxy-shaped vehicle that was seen with its engine running but no one inside on Clements' street, Colonial Park Drive, around the time of the shooting. Several witnesses told authorities they saw the vehicle. Presley said it might be a Lincoln Continental or a two-door Cadillac. Presley added a new detail about that clue: A man was seen driving the vehicle away from the neighborhood a short time later. Investigators are scouring databases to try to determine exactly what kind of car they're searching for, she said. Clements' death shook Hickenlooper, who spoke at an emotional news conference Wednesday. He repeatedly praised the corrections chief, calling Clements a "dedicated, committed, funny, caring expert at corrections" who tried to ensure that prisoners had adequate support before their release. "In many ways, Clements helped define what a public servant is," the governor said. He did his job quietly and intently. Clements became chief of the state's prison system in January 2011, according to Clements' online state biography. Before that he worked for 31 years in the Missouri Department of Corrections, where since October 2007 he had been director of adult institutions for the department, overseeing 21 adult prisons, the biography says. CNN's Jim Spellman reported from Colorado. Casey Wian and Paul Vercammen contributed to this report. Ashley Fantz wrote the story in Atlanta.