A series of late-night, window-rattling booms reported in two Southwestern Indiana counties has left investigators stumped.

Several residents of Vanderburgh and Warrick counties reported hearing loud booms and feeling vibrations on the nights of Jan. 7 and 8, spurring an investigation into the noises.

The Evansville Courier & Press reported Monday that local public safety agencies have found no concrete evidence about the source of the noises. The sounds had generated speculation that they might have been caused by explosions in nearby coal mines or produced by aircraft.

"There's a computer system that all the EMAs and all the public safety (agencies) can check for information, and we got nothing out of that," said Adam Groupe, deputy director of the Evansville-Vanderburgh County Emergency Management Agency. "They said it wasn't aircraft, and they said it wasn't mine explosions."

He said officials have received no additional complaints about the sounds since early Jan. 8.

Some residents likened the sounds to noises they are used to hearing periodically, such as normal air traffic from the airport, but that the noises were louder than normal during the two nights the booms were heard.

Others thought the sounds could be the result of someone shooting a gun at an exploding target, which authorities in Southern Illinois determined to be the source of similar reports there about a year ago.

None of the region's seismographs recorded activity when the booms were recorded, ruling out the theory that the unexplained booms were a geological precursor to major tectonic activity.