Caterpillar's shares are leading the Dow Jones index's retreat with a nearly 6% decline, putting the construction- and mining-equipment maker on course for its biggest drop since the end of July.

Caterpillar is the worst-performing stock in the index today. The shares are off about 24% since closing at an all-time high of $1,065 on June 30, but they are still up nearly 42% since the start of the year. Sales of Caterpillar's generators have been booming on demand from data-center developers that are using generators for backup power or as primary sources of electricity for power-hungry computer servers.

Caterpillar rivals Deere and CNH Industrial, which also make farm tractors and harvesters, are trading lower as well after the Federal Trade Commission and the Agriculture Department disclosed that they were investigating farm-equipment manufacturing and distribution for potential anticompetitive conduct.