Backers of a renewed investigation said a 2010 analysis of a recently enhanced audio recording concluded that someone may have ordered troops to prepare to fire during the campus protest. But the federal government said its review was inconclusive in determining whether the recording provided such evidence.

Assistant Attorney General Thomas Perez said last month on the issue of a command to fire that the government's analyst showed "no military-like voice commands to fire or otherwise were heard; rather, many of the words heard were probably uttered by several different individuals located closer to the microphone."

The original reel-to-reel audio recording was made by Terry Strubbe, a student who placed a microphone in a window sill of his dormitory overlooking the anti-war rally. A copy of the audio tape was found in a library archive in 2007.

The survivors asked Gov. John Kasich and Ohio Attorney General Mike DeWine to clear the way for an analysis of the tape by state crime laboratory investigators.