The mystery of what happened to an Ohio family – and why it happened – deepened this weekend when police found the bodies of a mother, her 11-year-old son and her best friend stuffed into rubbish bags and hidden in a hollowed-out tree.

Earlier last week, a tip-off led police to Tina Herrmann's daughter Sarah Maynard (13), who was found bound and gagged in a basement after a 10-day abduction. The girl was discovered alive but severely traumatised, just 10 miles from where she, her brother Kody, Tina (32) and her best friend Stephanie Sprang (41) went missing.

To detectives it appeared as if they had just vanished off the face of the earth until a tip led them to Sarah. Then, four days later on Thursday, the bodies of the other three were found 15 miles away in a wooded area.

Police were led to the bodies by unemployed tree trimmer Mathew J Hoffman (30) who was arrested at his Mount Vernon, Ohio, home where Sarah was discovered. "We were optimistic a few days ago that maybe there was a remote chance that these folks were possibly still alive," the Knox County sheriff David Barber said. "This is a homicide investigation now."

"This is probably the saddest day in Knox County history that I can remember," prosecutor John Thatcher said. "As elated as we were Sunday morning when Sarah was rescued, I think the tragedy today is just devastating."

A large amount of blood was found at Tina Herrmann's home after she failed to turn up for work at a Dairy Queen restaurant on Thursday 4 November. Police now believe the three were killed there, but are at a loss to know what the motive could have been.

Hermann's ex-husband Larry Maynard, father of the children, told WBNS-TV that a co-worker went inside the home and saw blood and things out of place before calling police.

Investigators have determined that Hoffman, who has been charged with kidnap, is not the ex-boyfriend of either Herrmann or Sprang, but they are trying to determine whether there is another connection, the sheriff said. "At this time, whether he's connected to the family or whether he connected himself to the family... a lot of that remains to be seen as the investigation continues," Barber said. It is believed he may have been watching the family.

Hoffman released information as to the whereabouts of the bodies through his attorney, but has refused to speak to police investigating the case.

At the Herrmann home yesterday, three purple, star-shaped helium balloons had been left in the yard and two bouquets of flowers were resting against a tree. A vigil that had been planned near the family's home to support search teams became, instead, a memorial. Rita Bates (60), who lives nearby, asked the question on the minds of many: "Why?"

At a vigil, Paul Manter, a friend of Hoffman's mother and stepfather said they "can't believe that their son did this". Manter attends the same church as the couple and said they had wanted to help with the search but he advised against it. "I wouldn't have suspected that he would do something like this," Manter said.

After the family's disappearance, police located Herrmann's truck on property owned by Kenyon College, about 50 miles northeast of Columbus. The discovery prompted officials at the college to temporarily lock the campus down. According to Barber, Hoffman was in the area where the truck was found and was questioned by law enforcement. No charges were filed and he was released.

In the days following the unexplained disappearance, investigators conducted multiple searches on land and from the air. Volunteers also came forward and offered assistance, searching wooded areas. "Everybody knows somebody that knows them, being a small town," neighbour Alicia Lawson said.

Police released few details of the investigation until last Sunday when a Swat team stormed the Columbus Road home where Hoffman was staying and discovered Sarah in the basement. "We were hopeful that we would find more than one [person], but our information was most definitely that Sarah was going to be in that house," Barber said. Police believe Sarah had been in the basement since she went missing 10 days earlier.

Hoffman has been held at the Knox County jail since his Sunday arrest on a kidnapping charge. He appeared in Mount Vernon Municipal Court on Tuesday through a video link from the jail, where he was wearing a green sleeveless shirt that revealed muscular arms. At a press conference later, Sheriff Barber called the garment a "suicide gown". Barber told reporters that Hoffman "gave indications to the jail staff and to the investigators" that he might harm himself, so Barber placed Hoffman on suicide watch "until a mental health expert says otherwise".

During his hearing, Hoffman mostly stared straight ahead, and yawned at one point. Authorities said more charges were expected. He previously spent six years in jail for arson.

Neighbours of Hoffman said they were wary of him. Dawna Davis (35), who lives next door, said she told her children to stay indoors when he was out. She said he moved in alone about a year ago and that a girlfriend lived with him temporarily with her son until about a month ago. "He would sit and listen to us up in a tree. He had a hammock and he would sit there and listen to us," she said. "He was just different. He was very different."

Investigators would not discuss details of the surviving girl's ordeal but have said she is with her father and doing well, considering the circumstances. "We're inspired by Sarah's bravery," Barber said. He said she was home at the time of the killings but added, "What she saw, I can't speak to that."