UnFound: 5 years ago, a Trib worker called off sick. His bones were found later, but what happened remains a mystery.

Daniel Niehaus was a quiet man and a reliable worker, which is why an unannounced absence alarmed his supervisors and co-workers.

Niehaus, an inserter at the Trib Total Media printing and distribution center in West Deer, had taken three sick days prior to Memorial Day weekend in 2013, then returned to work for one day before calling off sick again. He was supposed to return June 2 but never did, said Becky Callahan, administrative assistant.

“When I found out he didn't show up, I thought that was very odd because he always showed up for work and never called off,” Callahan said. “That's when we got concerned.”

Co-workers reported him missing to Penn Hills police and searched his neighborhood in the vicinity of Lime Hollow and Frankstown roads.

He was never found.

Almost a year later, a West View Water Authority employee found human remains on an Ohio River bank in Avalon. DNA from the bones was sent to a lab in Texas, which eventually matched the sample to one provided by Niehaus' family.

On July 25, 2015, more than two years after Niehaus' disappearance, the Allegheny County Medical Examiner's Office publicly confirmed that the bones were his.

On this, the fifth anniversary of Niehaus' disappearance, Trib Total Media continues to offer a $10,000 reward for appropriate information, depending on what authorities conclude is the cause of death.

Originally from Wheeling, W.Va., Niehaus started working for Trib Total Media in November 2005. Mailroom supervisor John Abraham described him as a good, dependable worker.

While on the job, he focused on his work to the exclusion of everything else, said Abraham, his immediate supervisor.

“The issue with Dan was that he was extremely quiet. Not that he didn't have friends because everybody liked him. But he didn't really talk to anybody,” Abraham said.

Callahan said she and Niehaus would exchange greetings on the mailroom floor but otherwise didn't make conversation.

“He always kept to himself,” she said.

Soon after the disappearance, Trib Total Media President and CEO Jennifer Bertetto and her husband, Production Director Keith Bertetto, organized a search with other employees. The search took them to a wooded area in Penn Hills and to Rodi Road, where they distributed flyers to businesses.

“Cleaning out his locker, it was just sad,” Jennifer Bertetto said.

Penn Hills police conducted numerous interviews with family members and friends, said Chief Howard Burton.

Niehaus' sister, Donna Milliken of Wheeling, could not be reached for comment.

After what it characterized as an extensive investigation, police never reached a conclusion on the cause or manner of death. The case is considered open but with all leads exhausted.

“We found him, so he's not a missing person. It's still just a question of how it happened,” Burton said. “We're not sure what happened.”

At the time the medical examiner's office announced the DNA match, it said the cause of death was undetermined.

Bertetto said Trib Total Media continues to offer the reward in the hopes that someone will come forward with information.

“Someone knows something,” she said. “I believe someone had to have seen something.”

Stephen Huba is a Tribune-Review staff writer. Reach him at 724-850-1280, shuba@tribweb.com or via Twitter @shuba_trib.