Rosenthal, 75, was found shot in the head on Sept. 22, 2009. He body was discovered in an enclosed patio outside the front door of her home at 647 Hilary Drive.
Tiburon resident and attorney Thomas Bertrand told Tiburon police that a donor or donors who wished to remain anonymous had offered a reward for information leading to the arrest and conviction of the suspect, police spokeswoman Margo Rohrbacher said.
Anyone with information about the slaying is urged to call (888) 898-5818 or (415) 499-7265.
To collect the reward, callers must be willing to identify themselves to Marin County sheriff's investigators and cooperate in the investigation and prosecution, Rohrbacher said.
Tiburon Police Chief Michael Cronin said this morning that the sheriff's office is handling most of the murder investigation.
Sheriff's detectives have traveled to New York City and Honduras where they interviewed people connected to the family of Rosenthal's late husband Ken, who died in January 2009, Cronin said.
The sheriff's office has spent 4,000 hours investigating the murder and is still conducting interviews, Cronin said.
"We're hoping someone who knows something we might not have heard before contacts us," Cronin said.
Rohrbacher said someone may have critical information about the case without knowing it.
"The information may seem trivial or unimportant, or a person may simply feel silly reporting it or uncomfortable about drawing police attention to someone," Rohrbacher said.
Shortly after the murder, the investigation focused on a party at a home adjacent to Rosenthal's residence the night before her body was found. Party guests were interviewed but no leads were developed, Tiburon police said.
Cronin said police do not currently have any persons of interest in the case.
Because Rosenthal's body was found inside an enclosed patio behind a locked gate, police believe that either Rosenthal let the killer inside, the killer had a key or that Rosenthal had stepped out of the enclosed area and the killer followed her back inside, Cronin said.