Authorities share update on person of interest after Michigan mom-of-8 vanished

Local and federal authorities are sharing an update on a person of interest in the case of a Michigan mom-of-eight who disappeared in December after she was last known to be driving to pick up an acquaintance.

Heather Kelley, a 35-year-old mother from Portage, has been missing for more than 100 days, law enforcement told ABC News, adding that they are treating this case as a "homicide investigation."

Authorities found Kelley's vehicle set ablaze on the side of the road on Dec. 11.

"The sheriff's office, as well as the Portage, Michigan, [Department of] Public Safety, have been working this in conjunction together as a homicide investigation that has led us into several different possibilities. But it leads us to one person of interest," Kalamazoo County Sheriff Richard Fuller III told ABC News in an exclusive interview. "That person of interest is still in jail on unrelated charges. But right now, we feel that the investigation continues us on a path toward a possible arrest at some point, down the road, of the suspect."

Fuller did not go into more detail about who the person of interest is -- but said no tip is too small.

"We've gotten in some pretty good information," he said. "We're headed in a path that we believe is a good path. The investigation seems that it's leading in an area that we are convinced is a solid connection."

The FBI has also offered a reward of $20,000 solely for information that leads to Kelley's location and discovery.

"I believe at this time we've had several tips so far in the last few weeks, since the reward did go up, and we are posting this reward up on mobile billboards throughout the area," said Peter Ellis, the resident agent in charge of the FBI's Detroit field office.

There is an additional $5,000 from local agencies.

Fuller told ABC News that investigators have DNA evidence from Kelley's car.

"We believe that given the spring weather and how it's warming up and more people are out traveling along the edges of the road in this area where we have searched fairly well, we believe that there's still a possibility that she is in that area and that springtime is when somebody is going to come across Ms. Kelley and then contact us," he said.

Other law enforcement officials said they are frustrated they can't find Kelley -- yet.

"This case is a really sad case," said Nick Arnold, the director of public safety for Portage. But, he said, "I'm very confident that we are going to be able to find Ms. Kelley and bring this case to a close."