Homeowners in San Francisco’s sinking and tilting luxury high-rise are suing the city and the building’s developer over claims they hid the building’s engineering flaws for years.

Twenty tenants who collectively paid about $75 million for condominiums allege the city and Millennium Partners knew as early as February 2009, before condominium units were sold, that the building was unstable, sinking more than four times engineers’ estimates while continuing to tilt in the earthquake-prone city.

San Francisco City Attorney Dennis Herrera sued the developer in August, claiming Millennium Partners alone knew that the 58-story tower was sinking faster than expected.

John Cote, spokesman for Herrera, wasn’t immediately available for comment on the new lawsuit filed Friday. Millennium Partners and its vice president in San Francisco, Sean Jeffries, couldn’t be immediately reached for comment.

The complaint in San Francisco state court is the second filed by residents seeking class-action status. The earlier case includes allegations of unfair business practices, creating dangerous conditions and breach of construction standards. The new complaint, alleging conspiracy to commit fraud, says the city’s Department of Building Inspection and the developer colluded to keep secret the condition of the multibillion-dollar project linked to redevelopment of a nearby city bus terminal.

“At present, the building has sunk at least 16 inches vertically and is tilting northwest 2 inches at its base and 15 inches at its highest point,” according to the complaint, which was filed by Jerry Dodson, a lawyer who is also a resident of the building.

Claiming the tower is vulnerable to damage in an earthquake, Dodson says residents weren’t told of the structure’s flaws until a 2016 homeowners association meeting.