At the same time, investigators were trying to retrace the steps of a convicted rapist charged with kidnapping the 22-year-old, Dru Sjodin.
Grand Forks County Sheriff Dan Hill has said Sjodin is probably dead - her blood was found in the suspect's car - but there has been no sign of a body. Thousands of volunteers and hundreds of National Guardsmen have searched two states for her, and the guardsmen were to continue searching through Sunday before winding down the effort.
"We have covered so much ground and with the weather getting worse, I'm not sure if any more searches would be effective," Sheriff's Maj. Mike Fonder said.
Sjodin, a University of North Dakota student from Pequot Lakes, Minn., was last heard from Nov. 22 as she spoke to her boyfriend by cell phone as was leaving the Grand Forks mall where she worked.
Prosecutors charged Alfonso Rodriguez Jr., 50, a convicted rapist released from prison earlier this year, with kidnapping her. Rodriguez says he is innocent.
Last January, four months before Rodriguez's release from prison, a Minnesota Department of Corrections psychologist warned in a risk assessment that the severity of Rodriguez's assaults on women appeared to be increasing and that Rodriguez's "victim pool" of women apparently chosen at random "suggests a need for broader notification of the public" than permitted by state guidelines.
Based on that review, among a group of documents related to the case that were released this week, Rodriguez was designated a Level 3 offender, a category reserved for those who show the highest likelihood to commit more sex crimes. But authorities decided against holding Rodriguez indefinitely for treatment.
Rodriguez is now jailed on $5 million bail. His attorney, David Dusek, has said Rodriguez is not talking to authorities.
A search warrant unsealed earlier this week in Crookston, Minn., where Rodriguez lived with his mother, showed about a three-hour gap between the time Sjodin disappeared and the time Rodriguez appeared at his home.
Authorities said they do not believe Rodriguez's story that he went to a movie near the Columbia Mall about the time Sjodin disappeared from the mall's parking lot. Police said the movie wasn't showing anywhere near the mall that day.
Volunteers and local authorities searched around Crookston and Grand Forks for several days before the National Guard was called in Friday, when temperatures fell to minus 10 degrees.
About 300 guardsmen searched several areas Friday, including under a bridge, where they drilled holes and lowered cameras into the Red River, between Grand Forks and Crookston.
"One of our officers said there was open water there at the time" of Sjodin's disappearance, Fonder said. "It's a process of elimination."
Maj. Mark Vavra, spokesman for the Minnesota National Guard, said the water was murky and the visibility was poor.
Other guardsmen used Humvees and all-terrain vehicles to search fields and rural roads. They covered about 400 square miles on Friday and planned to cover twice that during the weekend, Fonder said.
"We're hopeful that we can find Dru," he said.