Hagerstown City Police announced the reward Friday. The figure includes $5,000 from Crime Solvers plus $10,000 put up by Vernon Davis and Virginia Davis, two of the children of the late Wilda and Daniel Davis.
Nearly four months ago, first-degree murder charges in the Davises' slayings were dropped.
Russell Wagner's trial ended on Oct. 1 with a hung jury in Garrett County Circuit Court.
Washington County State's Attorney M. Kenneth Long Jr. considered a new trial but in late November, made the decision not to retry Wagner.
During the trial, prosecutors attempted to link Wagner to the slayings, saying Wagner was pressured into the crime by his landlord and the couple's son-in-law, Ted Monger, court records say.
Defense attorneys argued last fall that Monger and his friend, the late Charles Harmon, set Wagner up as the "fall guy" for the slayings, committed either by them or by someone working for them, court records say.
Wagner was indicted on two counts of first-degree murder and one count of burglary. Monger was never charged in the deaths.
The jury was unable to come to a decision about whether Wagner played a role in the slayings.
"We just hope somebody out there knows something and will come forward," said Vivian Davis, Vernon Davis' wife.
Vernon Davis said in November that family members find it hard to handle the fact that no one is being held accountable for the deaths.
Family members say the reward will be available only for the next six months.
To get the reward, a person would need to provide information that leads to the arrest and indictment of those involved in the crime, police said. Callers may remain anonymous.