The source close to the investigation told the AP about the knife discovery and also confirmed earlier media reports about the blood and a knife sheath being found in the parking lot of the mall where Sjodin disappeared.
Sjodin, 22, of Pequot Lakes, Minn., was last seen Nov. 22.
Alfonso Rodriguez Jr., 50, has been charged with kidnapping Sjodin. Evidence in the case, which was sealed by a judge after Rodriguez's arrest, should be released on Tuesday, the source said.
Rodriguez's car, a four-door, maroon 2002 Mercury Sable, was impounded by authorities before his Dec. 1 arrest in Crookston, Minn.
Rodriguez is a convicted rapist who has a history of attempted kidnapping with adult women, and has used a weapon in at least one assault. He was registered as a predatory offender, the classification for those believed to be at the highest risk of committing another sex crime.
Rodriguez was released from a Minnesota prison in May after serving 23 years for an attempted abduction in 1979. Wayne Swanson, who prosecuted Rodriguez, said Rodriguez tried to abduct a woman off the street, and stabbed her when she fought back.
The news comes as volunteers renewed their search for Sjodin.
Allan Sjodin said two weeks of fruitless efforts to find his daughter would not stop him from continuing the search. "I haven't lost any energy," he said.
"We're going to sit down and make sure we know where we're at, where we're going, and how we get there," Grand Forks Police Capt. Mike Kirby said.
Kirby denied police were giving up.
"No, I'm not prepared to say that we're giving up hope," he told CBS News Early Show National Correspondent Jon Frankel. "I mean, I've been in this business a long time and I understand the realities of a call like this, but by the same token Elizabeth Smart — I mean, anything could happen."