Law enforcement agencies also have conducted smaller searches based on a few of the nearly 900 leads authorities have received in the disappearance of Dru Sjodin, Grand Forks police Sgt. Michael Hedlund said.
"Twenty different law enforcement agencies have been working hard and long on the investigation," Sjodin's mother, Linda Walker, said on CBS News' Early Show Monday.
"We're gonna find you Dru, keep hanging on," Sjodin's brother, Sven, said at a news conference Sunday. "We're just around the corner from you and we know we're gonna find you."
Divers from as far as New York and Los Angeles searched the area's rivers and streams, and the U.S. Border Patrol searched by air.
Sjodin, 22, of Pequot Lakes, Minn., was last seen Nov. 22 after leaving work at the Columbia Mall in Grand Forks. Authorities believe she may have been abducted while she was talking to her boyfriend on her cell phone.
She is described as 5'5" tall, 130 pounds, with blue eyes and frosted blonde hair. She was last seen wearing a pink and purple V-neck blouse, black pants, black blazer, black loafers and a black pea coat, carrying a small black purse.
Although Sjodin remains classified as a missing person, "Everything I've heard, everything I've seen leads me to believe that she was (abducted)," Hedlund said.
Hedlund told CBS Radio News authorities are checking a variety of areas for a variety of reasons.
"I can't even begin to thank all of the people," Walker told Early Show co-anchor Hannah Storm. "I know there have been a lot of Dru's classmates that chose to be the entire time up there and not spend the holidays with their own families."
About 30 FBI agents are working on the case as are investigators with 20 different agencies from three states and the Canadian province of Manitoba, Hedlund said. Authorities are interviewing some "people of interest," he said, but there are no suspects.
A $140,000 reward is being offered for Sjodin's safe return.
Her unexplained disappearance has set nerves on edge in the Dakotas and Minnesota.
In Grand Forks, retailers said supplies of stun guns and pepper spray are selling fast.
"I'm sure there's a lot of scared young women right now," said Greg Washburn, manager of the camping supplies department at Cabela's.
Sjodin's family has set up a Web site www.finddru.com to appeal for volunteers, distribute posters and buttons, and otherwise get out the word on the search.
Anyone with information on Sjodin's whereabouts is asked to call police in Grand Forks at 701-787-8000, call the tips line at 701-780-8213, or e-mail: email@example.com.
"Dru is an extremely outgoing young lady," said her mother. "She has a gift of being able to communicate with anybody. She would make anybody feel comfortable being around her. Outgoing. A beautiful smile ... Just warms the room as she walks in."