Investigators from the FBI on Thursday concluded four days of “forensic digs” in Bedford County, Va., as part of a stepped-up investigation of the nearly 40-year-old case of two young girls who disappeared after walking to a Montgomery County shopping mall in 1975.

Officials did not say what, if any, evidence was found.

“Investigators are pleased with the progression of the search for the truth concerning the abduction and murder of the Lyon sisters,” the Bedford County Sheriff’s Office said in a brief statement. “This progression is the direct result of careful, methodical and thorough work this week.”

The forensic team, joined by inveestigators from Bedford, the Virginia State Police and the Montgomery County Police Department, searched land about 200 miles southwest of Washington. Detectives earlier identified two “persons of interest” in the case whose family once owned the land that is being searched, officials have said. Neither man has been charged.

Sheila Lyon was 12 and Katherine Lyon was 10 when they were last seen in March of 1975.