A 15-month-old girl was last seen in December. She was reported missing only this week.

"This is a complicated case," the Tennessee Bureau of Investigation tweeted Thursday about the disappearance of Evelyn Mae Boswell. "Right now, our primary focus is finding Evelyn."

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By David K. Li

A Tennessee sheriff said Friday he believes a 15-month-old girl — last seen in December but only reported missing this week — is still alive.

Law enforcement agencies across the state were on the lookout for the toddler and a gray 2007 BMW, with Tennessee stickers, because "individuals traveling" in that vehicle "have information regarding Evelyn Boswell’s whereabouts," the Tennessee Bureau of Investigation said.

The little girl's name was added to the state's database of missing children on Tuesday with a notation: "Evelyn was entered as a missing child on Tuesday February 18, 2020 but was reportedly last seen on December 26, 2019."

The last non-family member to see the girl was a babysitter on Dec. 10, authorities said. The girl's grandfather reported her missing to authorities this week.

"Finding Evelyn is our main concern and our top priority at this time," Sullivan County Sheriff Jeff Cassidy told reporters on Friday.

"This case is unlikely anything I've ever been involved in. We've had a child who has not been seen by the parents or certain family members in almost two months and was just reported this week."

In his meeting with reporters, Sheriff Cassidy was asked if he believes the girl is alive and he answered in the affirmative.

"I think so personally," he said. "Our main concern is to find Evelyn."

The sheriff did not elaborate on why he's optimistic about locating the missing girl. He revealed that Evelyn's mother has been talking to investigators but her stories haven't been consistent.

"She is cooperating but ... some of the information she's given us hasn't been really accurate," Cassidy said.

The girl's father, a serviceman stationed in Louisiana, is not married to her mother, according to Cassidy.

WCYB, an NBC affiliate in nearby Bristol, Virginia, spotted the baby's mother, Megan "Maggie" Boswell, Friday afternoon leaving the Sullivan County Courthouse. It was not clear what she was doing at the courthouse.

Boswell and another woman declined to answer any questions as they strolled to their car in the parking lot and drove away.

Evelyn is about 2 feet tall and weighs 28 pounds. She was last seen wearing a pink tracksuit, pink shoes, and a pink bow, officials said.

"Investigators are trying to determine why she wasn’t reported missing until this week," Tennessee Bureau of Investigation spokeswoman Leslie Earhart said in a statement to NBC News on Friday afternoon.

An exasperated Sheriff Cassidy said his investigators are also perplexed about the timing of missing person report.

"We have just as (many) questions as you all, even after four days," he said.

The Tennessee Bureau of Investigation on Thursday responded to a Twitter user who questioned why nearly two months went by before Evelyn was reported missing.

"This is a complicated case, and we appreciate your questions," the agency tweeted in response. "However, we aren’t in a position to talk about everything we know about the case publicly. Right now, our primary focus is finding Evelyn. We are working diligently to determine what happened to her."

Shamar Walters and Anthony Cusumano contributed.