The Tarrant County medical examiner's office determined that Sgt. Keith Shepherd suffered a pulmonary embolism. After falling, he was able to make it back to his own car where he was found Friday evening by colleagues who searched for him when he didn't return from his break during a shift at the county jail.
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Deputies initially thought he had been shot, leading law enforcement personnel to swarm the area around the county jail in Fort Worth.
But Fort Worth police, who are leading the investigation, said earlier Saturday that there was no evidence that a shooting occurred.
"We've just got to follow the facts until we understand what happened," Tarrant County Sheriff Bill Waybourn said in the hours after Shepherd was found.
It was initially believed that a suspect may be on the loose, but Waybourn had cautioned that there was no apparent ongoing threat to the public.
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"We're going to take it one piece at a time and see if we can find the answers," he said.
Investigators found blood inside and outside the car, and they had planned to review area surveillance video for insight into what happened.
Shepherd had worked for the sheriff's department for 19 years and was assigned to the jail.