A California couple left Oakland for a bike trip in South America in December has gone missing in Peru, it's reported that the US State Department has joined the search for the couple.

Garret Hand and Jamie Neal were last heard from on January 25 according to family members, when the couple was planning to travel from Cusco to Lima, Peru.

"Until someone shows me proof that she's not missing, I don't believe it," Jennifer Neal, one of Jamie's sisters.

KTVU reports the couple was last seen on January 26 taking a bus from Cusco to Lima and then getting in a cab.

"I really want to make this perfectly clear," Francine Fitzgerald, Hand's mother, said Monday at the family's home in Concord. "The generosity, love and caring showed by so many people -- family, friends, people in Chile and Peru -- it makes you hopeful. When I first realized they were missing, it was horrific, but that generosity, love, kindness, it has lifted our hope."

The website of US Embassy in Peru issued a travel warning for Americans. The warning posted on February 13 speaks about potential kidnapping threat.

"The Embassy has received information that members of a criminal organization may be planning to kidnap U.S. citizen tourists in the Cusco and Machu Picchu area," the statement said. "Possible targets and methods are not known and the threat is credible at least through the end of February 2013."

Pooja Jhunjhunwala, a spokeswoman for the State Department said the issued advisory had nothing to do with the couple's disappearance.

"At this time, we do not have a link between the two," she said.

Officials at the US Embassy in Lima are working with Peruvian authorities in search of the couple and the embassy is in contact with their families.