President Trump and Secretary of State Marco Rubio made their most detailed case yet on Wednesday for why they believe the American attack on Iran’s nuclear facilities dealt a fatal blow to the country’s ambitions, pushing back on the findings of a U.S. intelligence report and statements from international nuclear inspectors.

While Mr. Trump largely repeated his arguments that the facilities were “obliterated,” Mr. Rubio stepped in with a more detailed description of why he thought the Iranians were set back for years, rather than by only a few months as the preliminary Defense Intelligence Agency report said.

His argument centered on the belief that a “conversion facility” — which is key to converting nuclear fuel into the form needed to produce a nuclear weapon — was destroyed. The facility, in Isfahan, is where enriched uranium gas has been converted into solid materials, and ultimately a metal, that can be used to fabricate a nuclear bomb or a warhead.

Israel reported hitting the facility, and an associated laboratory for turning the fuel to metal, and The New York Times described the hit at the time. Independent analysts believe the plant was severely damaged.

“You can’t do a nuclear weapon without a conversion facility,” said Mr. Rubio, who serves simultaneously as interim national security adviser. “We can’t even find where it is, where it used to be on the map,” he added, speaking of the conversion facility. “The whole thing is blackened out. It’s gone. It’s wiped out.”

Satellite photographs show extensive destruction, but not until international nuclear inspectors are allowed on the site will it be possible to know what it would take to rebuild, on the site or elsewhere.