The portion of Bank of America’s derivatives portfolio that isn’t reported to regulators is not discernible in its public filings. A bank spokesman would say only that the percentage is small. A Morgan Stanley spokesman said “virtually all” of its trades were reported to American regulators.

The banks say that they aren’t trying to hide anything and that in some cases they are responding to demands from overseas clients who don’t want the United States government looking at their transactions. Even foreign bank regulators argue there’s no reason American law should apply to financial instruments held outside the United States.

“This problem, I think, is really driven more by regulators each wanting their own silo,” said Sheila Bair, a former chairwoman of the Federal Deposit Insurance Corporation. “I think the industry would be fine with some type of consolidated reporting.”

Mr. Dempsey, the lawyer, said, “What you have is a picture that has more clarity to it than what the regulators had in 2008, but you still don’t have maximum clarity.”

Regulators can still monitor risk for individual institutions. The Federal Reserve, for example, can ask for specific information about derivatives trades as it sees fit. But because the trades aren’t automatically reported, the regulator would have to decide which trades to ask about beforehand. Theoretically, the Fed could ask for banks to report every single trade, but the central bank hasn’t done that.

In October 2016, the Commodity Futures Trading Commission proposed a rule that would have closed the reporting loophole by requiring all American bank subsidiaries to report their derivatives exposure. It also would have subjected the subsidiaries to financial regulations that would have made derivatives trading less profitable.

The banking industry opposed the rule. After President Trump took office, it was never authorized.

Officials at the Commodity Futures Trading Commission acknowledge that there is a problem. The agency noted in an April paper that the current reporting “cannot provide regulators with a complete and accurate picture” of risks in the market.