Lupis said it appears the Jeep went into the opposite lane of travel and Schmidt was unable to return it to the northbound lane.
Police believe a southbound 2003 F-250 pickup truck driven by Donald Eugene Pitsnogle of Smithsburg collided with the Jeep on the passenger side.
Flowing Springs Road had to be shut down for about two hours after the 11:45 a.m. wreck, and Schmidt and Shields had to be cut from the wreckage, Lupis said.
Schmidt, who had a broken jaw, broken wrist and lower back pain, was flown to Winchester (Va.) Medical Center, Lupis said.
Schmidt, 26, of Shepherdstown, who also is a U.S. Coast Guard employee, later was transferred to Bethesda Naval Hospital in Bethesda, Md., Lupis said.
A spokesman at that hospital would not release Schmidt's condition Monday.
Pitsnogle, 46, was treated Sunday at Jefferson Memorial Hospital and released, a hospital spokeswoman said.
Shields worked at the National Vessel Documentation Center for several years, Crawford said.
The U.S. Coast Guard maintains documents relating to commercial and recreational boats in the United States at the center, said Director Thomas Willis.
Under federal law, the center is required to keep documents relating to ownership of boats and mortgages on them, Willis said.
Shields was a vessel documentation clerk, which involved renewing vessel records, Willis said.
"I know she was an enthusiastic worker. Her supervisor spoke well of her," Willis said.
Shields' twin sister, Stephanie, works at the U.S. Coast Guard's Operations System Center in Kearneysville, W.Va., and Schmidt works at the Coast Guard's headquarters in Washington, D.C., officials said.
"It's had a profound effect on three Coast Guard units," said Willis, referring to the accident.