The U.S. has intercepted encrypted communications believed to have originated in Iran that may serve as "an operational trigger" for "sleeper assets" outside the country, according to a federal government alert sent to law enforcement agencies. The alert, reviewed by ABC News, cites "preliminary signals analysis" of a transmission "likely of Iranian origin" that was relayed across multiple countries shortly after the death of Ayatollah Ali Khamenei. Khamenei, the supreme leader of Iran, was killed in a U.S.-Israeli attack on Feb. 28...