A senior Biden administration official said that U.S. spy satellites detected an explosion at the Kakhovka dam just before it collapsed, but American analysts still do not know what — or who — caused it.

Separately, a Norwegian seismic monitoring foundation reported that its equipment in Romania had detected two explosions from the direction of the dam. There was a weaker one at 2:35 a.m. on Tuesday, and then a stronger one, with movement equivalent to a magnitude 1 to 2 earthquake, at 2:54 a.m., about the time that the dam broke, the group said.

Experts say the dam, which was held by Russian forces, was probably destroyed by an intentional explosion within the massive structure. They say an explosion from the outside, like a missile strike, or a structural failure caused by earlier war damage and high water spilling over the top, were conceivable causes but far less likely.

Ukraine’s government says the only plausible scenario is that the Russians, who were in control of the dam, blew it up. Its security service on Friday released an audio clip of what it said was an intercepted phone call, in which a man it identified as a Russian soldier said “it was our sabotage group” who destroyed it, and added that the damage was “more than they planned.” The recording’s validity could not be determined.