Reversing its initial assessment, the CIA said a tremor detected in August near a Russian atomic test site was not a nuclear explosion.
In an opinion Tuesday that is likely to boost efforts to ratify a nuclear test-ban treaty, an independent panel appointed by CIA Director George Tenet dismissed suspicions that the "seismic event" was caused by a secret test.
The tremor was detected on Aug. 16, near the island of Novaya Zemlya.
The CIA, using spy satellites, eavesdropping and other information, concluded that Russia was conducting weapons related experiments on the island around the same time.
Later analysis, however, put the location of the tremor many miles offshore, reducing the likelihood that it was an explosion.
"The panel concurred in the CIA's assessment that nuclear weapons-related experiments were conducted by the Russians at Novaya Zemlya in mid-August 1997," the CIA said in a one-page statement issued Tuesday.
"The seismic event was almost certainly not associated with the activities at Novaya Zemlya and was not nuclear. However, from the seismic data, experts cannot say with certainty whether the Kara Sea event was an explosion or an earthquake."
Besides an earthquake, tremor could have stemmed from some sort of conventional detonation.
The CIA's initial report that the tremor probably originated from an explosion led U.S. officials to direct sharply worded inquiries at Russian diplomats over whether Moscow had secretly approved a violation of its pledge not to conduct nuclear weapons tests.