The Arizona sheriff leading the investigation into Nancy Guthrie’s disappearance recounted to PEOPLE the moments early in the case when he realized the 84-year-old’s disappearance was likely criminal in nature.

Pima County Sheriff Chris Nanos spoke to PEOPLE as the search for Savannah Guthrie’s mom Nancy Guthrie crossed its 100th day following her disappearance from her residence in Tucson, Ariz.

Nanos recalled being alerted about a missing person’s case around 2 p.m. over a text on Feb. 1.

A few minutes after the chief notified him about an 84-year-old woman missing from her house, Nanos says, he got another text that the scene of disappearance “doesn’t look right, something is wrong. We’re calling homicide.”

“The reason that homicide was called is because they are the investigative unit in charge of missing person. Search and rescue is their operational team that helps assist with these issues,” Nanos says.

Man seen in Nest camera outside Nancy Guthrie’s home the night of her disappearance.

Pima County Sheriff’s Department;

Nancy’s disappearance led to a massive, multi-agency search which has so far generated several leads — such as doorbell camera footage showing an armed suspect and a glove found near Nancy’s home.

Nanos says as the investigation continues, they are increasingly getting closer to answers.

“Everyday our DNA labs are working with our investigators and they’re coming up with different ideas and different thoughts of how to help them make this DNA work for us,” he tells PEOPLE. “How can we do more with what we have? And so that’s why I say it is — I think we’re getting closer.”

Nancy’s three children — Savannah, her brother Camron Guthrie and sister Annie Guthrie — have pleaded with whoever may be responsible behind her disappearance to return her.

“We will never stop looking for you,” Savannah wrote in a Mother’s Day Instagram post. “We will never be at peace until we find you.”

The FBI has offered a $100,000 reward for any information leading to Nancy’s return or an arrest in the case. Savannah and her family are offering a reward of $1 million.