That makes it impossible for city officials to aim scarce resources at the most problematic owners. And it makes it hard for researchers to know, for example, if property has become concentrated among fewer owners.

Because the stakes are so high and the spillovers significant, there has always been a public element to private property, said Susan Pace Hamill, a law professor at the University of Alabama who has written about L.L.C.s since the late 1980s.

“Should tenants have a right to know who they’re renting from?” she said. “Should cities have a right to know who owns the property? The answer is a resounding yes.”

L.L.C.s today hide what should be public information, she argues.

“I am quite disturbed by that,” she said. “Having participated in the evolution of L.L.C.s from their early days, I feel like they’re being abused.”

Wyoming passed the first L.L.C. statute in 1977 at the prodding of oil and gas interests, creating an entity with the liability protections of a corporation without the tax responsibilities of one. Hardly anyone took advantage of the tool, and few states followed until the I.R.S. blessed L.L.C.s a decade later. They then quickly became the entity of choice for all kinds of businesses, and by the mid-1990s, all 50 states had L.L.C. laws.

In Milwaukee, according to research by a Harvard doctoral student, Adam Travis, L.L.C.s have grown to about a quarter of the rental market in the two decades since they became legal in the state.

The original idea was never specifically about real estate, and anonymity wasn’t particularly the appeal. But over time, Ms. Hamill said, state laws have made it easier to conceal who’s behind L.L.C.s. So they have simultaneously grown more common and less transparent.