Oklahoma police said Thursday that they've interviewed a witness who drove by two girls minutes before they were shot dead along a country road.

"We can let you know and the shooters out there that we do have a witness that was there shortly before the shooting," Special Agent Ben Rosser of the Oklahoma State Bureau of Investigation said at an afternoon news conference.

His agency has interviewed the male witness and believes his story to be credible, Rosser said.

Rosser said it appeared that the gunman or gunmen used two guns to kill both Taylor Paschal-Placker, 13, and Skyla Whitaker, 11, on Sunday on County Line Road in Weleetka, Okla.

"Each girl was shot with both calibers," Rosser said. "In other words, if we have two shooters then each of the shooters shot each girl if we have two shooters."

Rosser stressed that it still isn't known whether there were multiple shooters, and investigators still don't have a motive, suspects or persons of interest in the case. Police returned to the scene of the crime Thursday to comb a riverbed and the area where the girls were found.