Crews are searching for the remains of 7-year-old Ramona Price, who vanished as she walked to her home in Santa Barbara County.
Police suspect Price was a victim of serial killer Mack Ray Edwards. He confessed to killing as many as 20 children before he committed suicide on death row in San Quentin in 1971.
Edwards was a heavy equipment operator who worked on freeway construction at the site along U.S. 101.
Santa Barbara police began focusing on the Winchester Canyon overpass where cadaver dogs alerted to a possible body. The excavation there is expected to take several days.
Police initially weren't allowed to dig up the roadway. On Tuesday, however, Caltrans began tearing down the overpass to build a new one, which gave officers a window to dig for the body.
"Three and half weeks ago, that time came. The phone rang, Caltrans called to say, 'If you want to search for your missing girl, now would be a good time,'" said Lt. Paul McCaffrey of the Santa Barbara Police Department.
Excavators removed six-inch layers of earth at a time.
"The dogs are alerting much more quickly and the handlers did tell us that their analysis it we're getting close to something," Santa Barbara Police Chief. Cam Sanchez said.
Officials said they hope to get down to the depth that would have been accessible to Edwards when he worked on the overpass.
Price's parents are deceased. She has an older sister said to still be heartbroken over her disappearance.
The Associated Press contributed to this report.