Clements, 58, was shot Tuesday night at his home in an upscale neighborhood in Monument, Colorado.
At least a half-dozen neighbors came forward in the wake of the shooting to say they had seen a dark, "boxy" car around Clements' house at the time of the murder. One witness said they saw the car parked and running Tuesday night, with no one inside, while another said they saw a white male driving the car, according to police.
Earlier today, investigators said they were looking at prison cases as a possible motive for the murder. And they said that Clements had posted an ad on Craigslist just hours before he was shot, advertising a used $1,200 mountain bike.
One prison case that drew speculation was Clements' denial about a week before his death of a request by a Saudi national, Homaidan al-Turki, to serve his prison sentence in Saudi Arabia.
Al-Turki, whose company sold recordings of speeches by American Islamic militant Anwar al-Awlaki, had been convicted in 2006 of false imprisonment and sexual assault.
Because of the media attention al-Turki's case has received, a spokeswoman for the Colorado Department of Corrections told ABC News that Al-Turki has now been removed from the general prison population for his own safety.