The lack of recent progress in the probe into Savannah Guthrie’s mother’s kidnapping may mean that investigators soon scale back their resources, sources with knowledge of the case told ABC News Friday.

Nearly three weeks after Nancy Guthrie vanished, investigators are finding their best leads are so far coming up empty.

Partial DNA recovered at the home is, for the moment, unidentified; technology companies and the FBI have recovered no additional video from her home security system; and investigators have been unable to associate a vehicle with her abduction, law enforcement sources told ABC News.

Right now, there are 400 investigators assigned to the case operating 24/7, the sheriff’s department has said. Given the seemingly sluggish progress, investigators believe the case would soon have to move into a new phase with fewer dedicated resources but a small task force focused on it long term, the sources said.

The Guthrie family is aware that the investigation might need to be transitioned and has been briefed on the fact that certain leads have not been panning out, the sources said.

“It has been over two weeks, and, in these types of investigations, you have to at one point move on to a long-term sustainable level of manpower,” said ABC News contributor Rich Frankel, who oversaw hostage negotiations in the FBI. “It is not a closed case.”

In a statement, the Pima County Sheriff’s Department said: “This remains an active investigation. As long as leads continue to come in, investigators will continue to follow up on them. Currently, several hundred personnel are assigned to this case from various law enforcement agencies and are actively reviewing thousands of tips.”

Jason Pack, a former FBI agent and current chief executive of Media Rep Global Strategies, said the gap between what a case looks like publicly at day 20 and what is happening inside the investigation can be enormous.

“The things most likely to break a case open are almost never the things being discussed in one-on-one press interviews. They are happening in labs, in Walmart security offices, in gun shops, in backend server logs, and in conversations between investigators who have been staring at the same timeline long enough that it might be time for a fresh set of eyes,” Pack said.

Investigators have a confirmed physical description of the suspect: a male, 5 feet 9 inches to 5 feet 10 inches tall, of average build.

They have traceable items, most significantly the 25-liter Ozark Trail Hiker Pack sold exclusively at Walmart.

Walmart has handed over purchase records covering all sales of that backpack across Arizona, law enforcement sources have told ABC News. Investigators can now work through every buyer.

Detectives have been canvassing gun shops across the Tucson area, showing the doorbell video to see if anyone recognizes it.

Pima County Sheriff Chris Nanos has publicly cleared all members of the Guthrie family, which allows investigators to focus resources outward.

The partial DNA recovered from inside Guthrie’s residence is currently being analyzed at a private out-of-state lab. Separating individual DNA profiles from a biological mixture is painstaking, precise work.

When the private lab finishes, those completed DNA profiles go back to the Pima County Sheriff’s Department.

From there, the FBI could input the profiles into its national database of offenders. The sheriff’s department has also said it is exploring options for investigative genetic genealogy.

The rewards for information leading to Nancy Guthrie or her captor now total more than $200,000.