DELPHI, Ind. (AP) – The FBI is using electronic billboards across Indiana to seek information in the killings of two teenage girls slain during a hiking trip, a federal agent said Thursday.

FBI Supervisory Special Agent Robert Ramsey said several companies are working with the agency to use their digital billboards to solicit tips in the killings of 14-year-old Liberty German and 13-year-old Abigail Williams.

He said the billboards are displaying photos of the teens, an image of a man considered the main suspect in the killings, and a telephone tip line to enable authorities to spread “the widest blanket possible” in generating tips.

Ramsey said he wasn’t sure how many digital billboards were being enlisted in that effort.

The Delphi girls’ bodies were found Feb. 14 in a rugged area near a hiking trail in the small city about 60 miles northwest of Indianapolis, a day after they vanished while hiking during a day off from school.

Police said Thursday they’ve received about 3,900 tips in the killings. About half of those tips were received after authorities released an audio clip Wednesday taken from a video German had recorded on her cellphone.

Investigators said that audio captured a man saying “down the hill” and they believe he participated in the killings. Authorities said they aren’t certain if the voice is that of a man shown in a blurry image also taken from a video on German’s cellphone.

Authorities have called that unidentified man the main suspect in the murders.

State Police Sgt. Tony Slocum said Thursday there have been no arrests in the killings and no one is in custody. He urged residents not “jump to conclusions” if they happen to see law enforcement activity.

State Police say the reward for information leading to an arrest or arrests in the case has grown to $50,000.

Call 844-459-5786 with information on the case. Tips can also be reported by emailing Information can be reported anonymously.