WASHINGTON -- Speculative traders' interest in crude oil has grown to the point that they now account for roughly 70% of all trading in West Texas Intermediate crude on the New York Mercantile Exchange, compared with 37% in 2000, according to an investigation by a congressional subcommittee that forms part of an escalating political assault on Wall Street's role in the run-up in oil prices.

The subcommittee's findings, based on data obtained from federal commodity-futures regulators, are the latest sign that Washington is...