U.S. and allied spy agencies have found no traces of telltale nuclear-related particles from North Korea's February 12 nuclear bomb test, leaving unresolved basic questions about the device's design, according to officials in the United States, Europe and South Korea, Reuters reported.

This lack of scientific evidence suggests that key questions may remain unanswered about the type of fissile material used in the test, which was detected by seismic sensors. It also leaves unaddressed questions about how far the North has advanced in its bomb design.

After the test, the U.S. Air Force Technical Applications Center in Florida dispatched WC-135 "sniffer" airplanes to look for traces of gas residue that could offer clues to the device's design, but those efforts apparently turned up empty, the officials said.

An Air Force spokesperson confirmed that the planes were dispatched but said no results from the missions could be released. A U.S. intelligence official said analysis from the tests "was continuing."

Based on seismic evidence, both officials and private experts say there is little doubt that the North Korean device was several times more powerful than those tested in 2006 and 2009.