Experts cautioned that the available evidence was very limited, but they said that an internal explosion was the likeliest explanation for the destruction of the dam, a massive structure of steel-reinforced concrete that was completed in 1956. And local residents reported on social media that they heard a huge explosion around the time the dam was breached, at 2:50 a.m.
A blast in an enclosed space, with all of its energy applied against the structure around it, would do the most damage. Even then, the experts said, it would require hundreds of pounds of explosives, at least, to breach the damn. An external detonation by bomb or missile would exert only a fraction of its force against the dam, and would require an explosive many times larger to achieve a similar effect.
"You're going to be limited in how much a warhead can carry," said Nick Glumac, an engineering professor and explosives expert at the University of Illinois at Urbana-Champaign. "Even a direct hit may not take out the dam."
"This takes a significant amount of energy," he said. "You think about the forces on the structure in operation — they are immense. You have the water force, which is massive. This is not like holding on by a thread; these things are tough."
Over more than a year of heavy fighting, the Kakhovka dam had been damaged repeatedly, and each side has accused the other of shelling it. The Russians captured it last year when they advanced to the Dnipro and beyond, but months later the Ukrainians pushed Russian forces off the west bank, turning the river — and the dam — into part of the boundary between the warring sides. The Russians held onto the dam, itself.
It is not clear, though, that the kind of damage the dam had sustained was anywhere near enough to cause it to break down.
"Dams do fail; it's absolutely possible," said Gregory B. Baecher, a professor of engineering at the University of Maryland and member of the National Academy of Engineering, who has studied dam failures. But, he said, "I look at this and say, 'Gosh, this looks suspicious.'"
In August, a Ukrainian rocket struck the roadway on top of the dam. In November, as Russian forces withdrew across the river, an explosion destroyed part of the roadway; after that, images verified by The New York Times showed damage to some of the sluice gates that let water through. But there was no indication of damage to the underlying structure.
Since November, the gantry cranes that open and close the sluice gates have barely moved, though it was not clear if they had not been working. That led first to record low water levels and then, as winter snowmelt and spring rains flowed into the reservoir upstream, to a
30-year record high water level.
Since early May, water has risen above the gates and crested over the top of the dam. Satellite images taken last week showed more of the roadway gone; whether it was washed away by the flow of water or destroyed in a strike is unclear.
Some dams have collapsed because of unusually heavy water flows "overtopping" them. "Normally, such a failure would start on the earthen part of the dam, on either bank," said Professer Baecher.
But photos and videos show that the Kakhovka dam was first breached in the middle, next to the power plant adjoining the Russian-held bank. Both ends of it appeared to be intact at first, though as the day went on, more and more of the dam collapsed.
A combination of damaged sluice gates and high water might tear away a few gates, but would not be expected to rip apart so much of the dam, the professor said.
Ukraine on Sunday appeared to begin a long-expected counteroffensive against Russian forces, and its officials said that Moscow blew the dam to hinder their advance by causing flooding and removing the only remaining river crossing between the enemies. It is not clear, though, whether Ukraine's plans call for a major crossing of the lower Dnipro.
Ukrainians questioned why they would want to destroy their own infrastructure, towns and farms, while noting that those have been frequent targets in the brutal Russian conduct of the war. Moscow wanted to "show they are ready to do anything" if Kyiv aggressively pursues its counteroffensive, said Roman Kostenko, chairman of the defense and intelligence committee in Ukraine's Parliament. "They do everything to stop our counterattack."
Mr. Peskov, the Kremlin spokesman, claimed that Ukraine had destroyed the dam to cut the flow of water through a canal from the Dnipro to the Crimean Peninsula. After Russia illegally annexed Crimea in 2014, Ukraine halted the flow, but Russia restarted it last year after taking the dam.
Other Russian officials claimed the attack was meant to support a Ukrainian offensive that they said was sputtering — possibly to allow Kyiv to reposition some forces, or to have floodwaters push back Russian artillery near the river.
Some Western military analysts struck a cautionary note about trying to assign blame quickly, or even about saying whether the dam collapse was intentional.
"It's too early to tell," said Michael Kofman, the director of Russian studies at CNA, a research institute in Arlington, Va. The disaster, he said, "ultimately benefits nobody."
