Ohio Democratic Sen. Sherrod Brown said Wednesday he will continue pressing banks about their commodities holdings and will likely hold another hearing in September.

Brown, whose banking subcommittee held a hearing on the issue Tuesday, said on a press call that he still needs more information from regulators including the Federal Reserve and the Commodity Futures Trading Commission to know whether banks and other companies are doing anything illegal.

“They are manipulating the system, but I don’t know yet whether they’re violating the law,” Brown said.

Brown said the CFTC has not been as forthcoming as he would like. He called on the CFTC to crack down on anticompetitive practices and stop the warehouse bottleneck that allows aluminum traders and producers to charge more.

“There are other commodities operations doing these things and that’s why we want the CFTC involved in this too. They haven’t been as forthcoming as we’d like.”

Brown didn’t say who would be called to testify at the next hearing.

Senators and witnesses at Tuesday’s hearing said banks’ ownership of oil pipelines, metals warehouses and other commodity assets poses a significant risk to the financial system and imposes extra costs on companies and consumers.