Athens-Clarke police confirmed Monday that authorities recovered Zinkhan's passport from the 2005 Jeep Liberty, which, along with Zinkhan, had been the subject of a nationwide search since an April 25 triple-shooting at Athens Community Theater.
Police say Zinkhan, 57, opened fire at a theater gathering that afternoon, killing his wife, Marie Bruce, formerly of Augusta, and two others, Tom Tanner and Ben Teague.
Finding the passport leads investigators to believe Zinkhan still is in the United States, Athens-Clarke police Capt. Clarence Holeman said Monday.
But authorities aren't limiting the scope of their search yet, because Zinkhan still could try to leave the country illegally, said Jim Fullington, special agent in charge of the Georgia Bureau of Investigation's Athens office.
"It means he hasn't left the country under his name," Fullington said Monday of the recovered passport. "We're not ruling out anything with him."
Investigators have finished processing and taking evidence from the vehicle, which was being housed Monday at the Athens GBI office.
Neither Holeman nor Fullington would identify what other items they found, though Fullington said the items wouldn't help them locate Zinkhan.
"None of that tells us where he is or where he isn't," Fullington said.
Investigators think Zinkhan fled after pushing the Jeep into a ravine in Western Clarke County, a few miles from the house near Bogart where he and Bruce lived.
Authorities searching more than 1,000 acres in the immediate area Friday and Saturday came up empty.
Investigators have accounted for all of the vehicles Zinkhan was known to have driven and to which he was known to have access, Fullington said.
Authorities haven't speculated much on what motive Zinkhan - an aloof but successful professor - may have had in the murders, other than to say his wife planned to divorce him.
Until recently, Zinkhan apparently was making plans for his future.
The former UGA marketing professor had been expected to travel to a university in Amsterdam - where he also has taught - on Saturday. The university's dean had invited Zinkhan there to set up a marketing department, and the school booked his plane ticket.
On Saturday, authorities staked out Hartsfield-Jackson Atlanta International Airport but saw no sign of the fugitive professor.
Zinkhan also may have been looking for a new job in the months before the shooting.
In March, he interviewed for a position at the University of Alabama's business school, university spokeswoman Cathy Andreen said Monday.
"He was not offered a position," Andreen said.
Zinkhan's profile at the online social networking site Facebook shows he recently became a "fan" of Clemson University, where one of the shooting victims, Tanner, worked as an economist.
A Clemson University spokesman said Monday he had found no reason to believe Zinkhan had applied for a job there. The university also currently has a hiring freeze, so Zinkhan likely would have found no openings if he had looked, said spokesman John Gouch.