(Reuters) - Investigators hoping to solve the disappearance of an 8-year-old girl more than 66 years ago were digging Tuesday in a vacant lot amid single family homes in Janesville, Wisconsin, based on a tip that she may be buried there, authorities said.

The tipster pointed out the site and trained dogs indicated there may be human remains there, lending credence to the tip and leading authorities to launch the dig on Monday, investigators said. They have declined to identify the man who gave the tip.

"Whether or not it is her, it is hard to say," Jefferson County Sheriff's Captain Jerry Haferman said. "There is a good chance that there is something there."

Georgia Jean Weckler was last seen on May 1, 1947, when she was dropped off by a friend's mother at the end of the driveway to her family farm about half-mile from her house, Haferman said.

"She was going to pick flowers for a May Day basket and the friend recalled looking back and seeing her get the mail out of the mailbox and that was the last she was ever heard of," he said.

Thousands of people searched for her and several reported seeing a dark-colored 1936 Ford sedan in the area, Haferman said. Authorities have looked into several tips and even confessions over the years, but none were confirmed, he said.

The tip turned in to Rock County Sheriff's detectives and Janesville police on July 30 was the first since a tip was checked out in 1999, Haferman said.

The latest search is in an area that was partially wooded and rural in neighboring Rock County, south of the Weckler farm, when she went missing. It was annexed decades later into Janesville and the site is due soon to have a house on it.

No remains were found on Monday by the 25-30 people participating, said Rock County Sheriff's Captain Todd Christiansen. The dig resumed on Tuesday and could take several days.

Officials mapped out 24 grids about 10 feet by 10 feet each and have been digging with hand tools and sifting the earth, Christiansen said. They plan to excavate about four feet down during the search, he said.

(Reporting by David Bailey; Editing by Nick Zieminski)