Radioactive gases that may have come from North Korea’s nuclear test in February have been detected 1,000 kilometers away in Japan, a global monitoring body said Tuesday.

The Comprehensive Test Ban Treaty Organization (CTBTO) said it had detected radioactive noble gases in Takasaki, Japan, about 1,000 km (620 miles) from the test site, backpedaling from its statement in mid-March that any traces would be highly unlikely to be picked up.

“Two radioactive isotopes of the noble gas xenon were identified, xenon-131m and xenon-133, which provide reliable information on the nuclear nature of the source,” the Vienna-based monitoring body said.

While it was unusual to detect radioactive gases more than seven weeks after a test, the traces were “consistent with a nuclear fission event”, CBTO said, referring to the North’s underground detonation on February 12.

Lower levels were also detected at Ussuriysk, Russia, one of CBTO’s several hundred monitoring stations worldwide, reports Reuters.

The CBTO said, however, that the discovery couldn’t help determine whether Pyongyang used plutonium or uranium in its February test which yielded a stronger blast than its 2006 and 2009 tests, reports the Japan Times.

Pyongyang used plutonium in its first two tests. Any discovery that it used highly enriched uranium for its third test would mark a significant technological step for the secretive state, raising concerns that the regime could very well fit an atomic weapon on a missile.