パウエル米国国務長官の来日中、事件は起こった。

、。

The incident occurred during US Secretary of State Powell's visit to Japan.

Literal

Powell US Secretary-of-State [genitive-の] visit-to-Japan during, incident [topic-は] occurred.

News-report register. 国務長官 ('Secretary of State') is the Japanese title for the US position. The full frame パウエル米国国務長官 ('Secretary of State Powell of the United States') places the name-title-country in the news-typical order: [name] + [country] + [title]. 来日中 ('during the visit to Japan') is a compact temporal frame using the ~中 ('in the middle of') suffix on 来日 ('coming to Japan'). The full phrase 来日中 means 'while [the subject was] visiting Japan.' 起こった ('occurred') is past tense of 起こる, used for events in past reporting.