A 26-year-old Minnesota mother of two went missing a week ago after dropping her children off at day care – and police are calling her disappearance “involuntary” and “suspicious.”

Madeline Kingsbury was last seen March 31 when she dropped off her two kids, ages 5 and 2, at their day care in Winona with their father, Police Chief Tom Williams said, KARE 11 reported.

Kingsbury, a clinical research coordinator at the Mayo Clinic, went back to her home about 8:15 a.m. in a 2014 Chrysler Town and Country minivan – but never showed up for work, he said.

The children’s father, who has not been named, told police he left the residence in the dark blue vehicle around 10 a.m. and that when he returned later in the day, Kingsbury wasn’t home.

Williams said it was “extremely out of character” for Kingsbury to not show up for work, adding that there was nothing to suggest she left on foot or in another vehicle.

Investigators obtained surveillance video of a van matching the description of the couple’s vehicle traveling along County Road 12 and Highway 43 in Winona County and then in Filmore County between 10 a.m. and 1:30 p.m.

Authorities said they could not share information about who was behind the wheel, ABC News reported. The couple’s van was later found in their driveway.

Adding to the mystery, police found Kingsbury’s wallet, cellphone and ID, as well as the jacket she wore that day, inside the house, the chief said.

“Based on all of this, we believe Maddie’s disappearance is involuntary, suspicious and we are all concerned for her safety,” he said.

“We know that the children were dropped off at the day care and that the van returned home,” Williams added.

Police said they have not identified any suspects or persons of interest.

The missing woman’s older sister, Megan Kingsbury, said the family is offering a $50,000 reward for information on her whereabouts.

“Please help us find Madeline. The children need their mother. We need our daughter, our sister, our aunt and our best friend back,” she said.

Megan described her sister as a “hardworking and dedicated mother.”

Williams said the couple’s children are safe.

“We are very hopeful we will find her,” he said as authorities announced a mass search on Friday using volunteers over the age of 18.

Hundreds of volunteers from all over the state and neighboring Wisconsin came out to help find Kingsbury Friday, CBS Minnesota reported, adding that the turnout was more than officials anticipated.

“This is a sad story. It takes a community to actually go out and have some success, so we’re hoping we can find some answers today,” volunteer Amy Scheevel told the station.

“Anything I can do to try to get answers for her family and friends … We’ve never had anything this big close to home,” added Danica Bauer.

Kingsbury is described as 5 feet 4 inches tall and about 135 pounds. She has brown hair and hazel eyes.

According to her LinkedIn page, Kingsbury earned a bachelor of science degree in public health from Winona State University and a master’s from the University of Minnesota School of Public Health.

She is a clinical research coordinator at the renowned Mayo Clinic in Rochester, Minnesota.

Kingsbury recently posted a message on the site saying she was “searching for remote research related or public health psitions.”

The state Bureau of Criminal Apprehension is assisting in the investigation.

Anyone with information is asked to call Winona police at 507-457-6288 or contact Crimestoppers Minnesota at 1-800-222-TIPS (8477).