Why did US govt order Claude maker to restrict access to its most powerful AI models?

Anthropic globally suspended access to its most powerful AI models, Claude Fable 5 and Mythos 5, over a US government order. The order directed Anthropic to restrict the models' access for foreign nationals within and outside US over national security concerns, without specifying what those concerns were. Anthropic said government has given only "verbal evidence" of the models' narrow jailbreak.