U.S. regulators disclosed a broad nationwide probe into potential oil-market manipulation and said they are expanding surveillance of energy markets.

The move Thursday by the Commodity Futures Trading Commission, including its unusual announcement of an investigation in progress, comes after crude-oil prices topped $130 a barrel last week and tested all-time highs. On Thursday, light, sweet crude for July delivery settled $4.41, or 3.4%, lower at $126.62 a barrel on the New York Mercantile Exchange.

Lawmakers...