A deliberate explosion inside the Kakhovka dam, on the front line of the war in Ukraine, most likely caused its collapse on Tuesday, according to engineering and munitions experts, who said that structural failure or an attack from outside the dam was possible but less plausible explanations.

Ukrainian officials blamed Russia for the failure, noting that Moscow’s military forces — which have repeatedly struck Ukrainian infrastructure since invading last year — controlled the dam spanning the Dnipro River, putting them in a position to detonate explosives from within.

“It was mined by the Russian occupiers. And they blew it up,” President Volodymyr Zelensky of Ukraine wrote on social media.

Russian officials, in turn, blamed Ukraine but did not elaborate on how that might have been done.

Experts cautioned that the available evidence was very limited, but they said that an internal explosion was the likeliest reason for the destruction of the dam, a massive structure of steel-reinforced concrete, completed in 1956. And local residents reported on social media that they heard a huge explosion around the time the dam was breached, at 2.50am (local time).

A blast in an enclosed space, with all of its energy applied against the structure around it, would do the most damage — and even then, they said, it would require hundreds of pounds of explosives, at least, to breach the damn.

An external detonation by a bomb would exert only a fraction of its force against the dam and would require explosives many times larger to achieve a similar effect.