Their disappearance prompted authorities to lock down Kenyon College on Thursday night, after one of the missing person's vehicles was found parked near campus, 10TV News reported.
Deputies were at Tina Herrmann's home on King Beach Drive since 5 p.m. Thursday. Herrmann, 32, and her two children, vanished on Wednesday, along with Herrmann's friend, Stephanie Sprang, 10TV's Jason Frazer reported.
According to Knox County Sheriff David Barber, the inside of the house was in "unusual condition." He said he was surprised that children and adults would be living inside.
The children, Sarah Maynard, 13, and Kody Maynard, 11, were not in school on Thursday.
The Knox County Sheriff's office issued an endangered missing child advisory shortly after 4 p.m. on Friday.
It is not an Amber Alert, but the sheriff's office said the children are believed to be in danger. A check of the residence revealed evidence that the occupants were possibly a victim of a crime, the sheriff said.
Investigators said blood was found inside the home in two locations, but Barber said he believes it was from some type of an injury and does not necessarily mean foul play was involved.
Investigators were also monitoring Herrmann's cell phone and bank records.
Officials from the FBI, Knox County Sheriff's office, Ohio State Highway Patrol and the state Bureau of Criminal Investigation and Identification are involved in the search, Frazer reported.
Herrmann's pickup was discovered late Thursday night near the Kenyon College campus in nearby Gambier.
"It makes no sense to me," said Larry Maynard, the father of the missing children.
Larry Maynard said that he spoke with one of Herrmann's co-workers, who went to her house and peeked inside her windows after not showing up for work.
According to Barber, Herrmann worked at Dairy Queen, located at 1600 Coshocton Ave., in Mount Vernon.
"I guess (the co-worker saw) some beer cans and some things that looked kind of out of place, and she knew that something was definitely not right," Larry Maynard said. "My ex kept a pretty clean house. She went and entered the home and when she went into the home, she saw some blood in the home and -- at that point -- she turned around and left."
According to Larry Maynard, the co-worker then called 911. He said that Herrmann's live-in boyfriend, Greg Borders, contacted him after she was reported missing.
Borders said the couple was having trouble, but said he knows nothing and has done nothing wrong, 10TV's Tanisha Mallett reported.
He said he last saw Herrmann and her children around 4 a.m. Wednesday, before he left for work at a Target distribution center in West Jefferson.
He said he and Herrmann talked briefly on the phone shortly before 5 a.m. and received a text message from her at 11 a.m. Borders said that was the last time he heard from her.
Borders said he had no idea Herrmann never made it to her job because he spent the night with a friend.
"So I spent the night there and we went golfing and probably about noon I got a call from my mom asking me if I knew where Tina was," Borders said. "The way things are going down and then to have CSI out here as long as they have been in the house something had to have happened."
The couple had been together for six years. Borders said their home was going into foreclosure; they were talking about splitting up, and had been arguing.
He said he would never hurt Herrmann or her children.
Larry Maynard said he is not so sure.
"The boyfriend, the live-in boyfriend, Greg, I mean I don't believe anything he says," Maynard said. "My kids are my world. It's like somebody had ripped my heart out of me. I don't want to believe that something has happened to my children, but in my heart, I know there's something that's happened to them."
Sarah M. Maynard is 5 feet tall, weighing 90 pounds, with blonde hair and green eyes.
Kody A. Maynard was described as 4 feet, 10 inches tall, weighing 70 pounds, with brown hair and hazel eyes.
Stephanie L. Sprang is 5 feet, 3 inches tall, weighing 110 pounds, with blonde hair and green eyes.
Tina R. Herrmann was described as 5 feet, 6 inches tall, weighing 122 pounds, with sandy blonde hair and blue eyes.
Investigators said they are treating this as a missing persons case, and they have no suspects and no persons of interest.
The discovery of Herrmann's pickup prompted the Kenyon campus to be locked down at about 11 p.m. Friday. Students were asked to stay inside campus buildings. The lockdown was lifted Friday at 7 a.m.
Anyone with information about the case is asked to contact the Knox County Sheriff's Office at 740-397-3333.
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EDITOR'S NOTE: This story has been updated. Please click here for the most recent information.
November 12, 2010: 4 Missing From Knox Co.; Discovery Of Pickup Prompts Kenyon College Lockdown