One hundred days is an agonizingly long time for a family to go without answers about their missing loved one. It's also long enough for the investigators working the case to feel the grind.

At the 100-day mark, the investigation into the disappearance of Nancy Guthrie could hang on a strand of DNA that's still being evaluated by scientists at the FBI laboratory in Quantico, Virginia, in hopes of identifying her abductor.

Guthrie, the mother of "Today" show co-anchor Savannah Guthrie, was reported missing Feb. 1.

The DNA recovered from Guthrie's home in Tucson, Arizona, was first sent by Pima County sheriff's detectives to a private laboratory in Florida for analysis. Weeks later, the DNA was shipped to the FBI's state-of-the-art crime lab in Quantico, where law enforcement sources tell CBS News the work is ongoing. 

Investigators have not publicly confirmed details about the sample or exactly where it was found.

"An item containing DNA such as a strand of hair found somewhere in a house is one thing, but a strand of hair near the victim's last known location, such as in her bed, would be a high priority for the FBI," said Lance Leising, a former FBI supervisory special agent in Arizona.

The painstaking process of DNA analysis, including building out family trees, doing forensic comparisons and elimination work "takes far longer than television crime dramas would have people believe," said retired FBI Supervisory Special Agent Jason Pack. "That kind of work is slow because it has to be right." 

Reached by phone on Monday, Pima County Sheriff Chris Nanos declined to comment on whether the DNA was from a strand of hair, bodily fluid or from another source, citing the need to maintain the integrity of the case.

"It would be highly inappropriate of me to speak to the evidence. We have to keep the integrity of this case. If we make an arrest, that individual has the right to a fair trial, [so] I can't sit here and address all of that," Nanos told CBS News. "We are working hard with all of our partners to resolve this case, and we will."

If bodily fluid was found anywhere in the house, it would be a top priority for the FBI, since it's easier to get a full DNA profile from bodily fluid. Skin cells often yield lower levels of DNA, making it hard to pinpoint an individual. 

If the source of the DNA is a hair sample, any full hair follicles that include the root would be much more likely to contain high-quality DNA than just a strand of hair.  

Sources close to the investigation told CBS News that even a low-level sample can be enhanced by scientific techniques in the lab, but the process could be lengthy.

Earlier in the case, another DNA sample turned out to be a dead end. The sheriff's department recovered DNA from a set of gloves found about 2 miles from Nancy Guthrie's house, but eventually traced them back to a nearby restaurant worker who had no connection to the investigation.

In a statement provided to the media on Monday, a spokesperson for the Pima County Sheriff's Department said, "As we reach the 100-day mark in this investigation, scientific evidence processing and digital media analysis remain ongoing." They added that investigators are "actively pursuing leads and tips," and continued to encourage anyone with information to come forward. 

Doorbell camera video

Aside from DNA, the other promising lead that generated a huge amount of attention was video from a Ring doorbell camera, which captured images of a suspect wearing a ski mask, gloves and a backpack at Guthrie's front door the night she vanished.

The FBI said the video was recovered from "residual data located in backend systems."FBI Director Kash Patel said in a recent podcast that he spoke directly to Google leadership in order to obtain it.

The FBI quickly put it out to the public on Feb. 10 in hopes someone would recognize the man in the video. In the days that followed, tips flooded in and investigators carried out several searches that led nowhere.

So far the FBI has not released any more video to the public, but it continues to urge people to share any information that might be tied to the case to the FBI tip line.

The FBI and sheriff's department have already received thousands of tips. Phone data, surveillance video, GPS data, app and cloud records and social media accounts also create mountains of information investigators must sort through piece by piece. 

"Sometimes the biggest breakthroughs come not from dramatic confessions or Hollywood-style moments, but from patient analysis and one tiny inconsistency that finally breaks a case open," Pack said.

100 days — and the path ahead

Law enforcement agents who've worked missing child cases, violent crime investigations and hostage situations know that the public often interprets silence for inactivity. 

"That is not how major investigations work. The cameras may move on. The investigators usually do not," Pack said. "A hundred days feels like forever to a family. In investigative terms, it can still be early."

Behind the scenes, the lead detective and case agents assigned to the Guthrie case have spent the last 100 days doing the slow, frustrating and exhausting work the public never sees like, re-interviewing witnesses, sorting through old tips and information with fresh eyes, and running down new tips. 

"They're looking for the one detail that did not fit the first time around," Pack said.

On Mother's Day, Savannah Guthrie released a statement on social media, writing, "We will never stop looking for you. We will never be at peace until we find you."

She continues to ask the public for help. 

The FBI is offering a reward of $50,000 for information leading to the recovery of Nancy Guthrie or the arrest and conviction of anyone involved in her disappearance. The Guthrie family has offered an additional reward of $1million for information. Local Crime Stoppers is offering $2,500 plus a $100,000 donation from Milwaukee attorney Michael Hupy.

"Someone knows something that can make the difference," Guthrie wrote in her Mother's Day post. "Please keep praying. Bring her home."