The Securities and Exchange Commission couldn't nail Steven A. Cohen for insider trading. So now it's going after him for (allegedly) failing to prevent it.

It's a legitimate basis for a claim that he violated securities laws, after years of investigations by federal regulators and the Justice Department. It just isn't all that satisfying -- a fallback plan that suggests the authorities felt compelled to accuse Cohen of something, anything so they would have something to show for their efforts.

The SEC's enforcement division, in an administrative complaint today, said that Cohen "failed reasonably to supervise two of his senior employees, who engaged in insider trading under his watch." The key assumption here, of course, is that the two employees, Mathew Martoma and Michael Steinberg, actually engaged in insider trading. Both have been charged criminally and are awaiting trial after pleading not guilty.

The complaint said both portfolio managers "provided information to Cohen indicating that they may have had access to inside information to support their trading." The SEC said the information was "highly suspicious" and "should have caused any reasonable hedge fund manager in Cohen's position to take prompt action to determine whether employees under his supervision were engaged in unlawful conduct." Cohen, 57, failed to do so and ignored lots of "red flags" in the process, the agency said.

In short, the SEC's lawyers accused Cohen of being uncurious in the extreme.

SAC, as a firm, already has paid more than $600 million to settle the commission's insider-trading claims. Now the agency is seeking to bar Cohen from overseeing investor funds. So if he lost the case he could still manage his own considerable wealth, just not other people's money. That would sting, but it wouldn't keep him from living the billionaire lifestyle to which he is accustomed.

From here, the case against Cohen will be assigned to an administrative-law judge. It could be a long slog. Until we know what becomes of the allegations against Martoma and Steinberg, there's not much good in speculating about the SEC's case against Cohen.

One suggestion for the SEC: In the event Cohen does decide to settle the claims against him, don't dare let him off with one of those awful "neither-admit-nor-deny" deals. The old ways of doing things simply won't suffice here.

(Jonathan Weil is a columnist for Bloomberg View. Follow him on Twitter.)