Opponents of the Indian casino being built next to Rohnert Park have sued the city over its recent agreement with the tribe to widen Wilfred Avenue.

The suit filed Tuesday in Sonoma County Superior Court challenges the city's conclusion that the project is exempt from state laws requiring environmental studies.

Wilfred Avenue is the northern border of the Federated Indians of Graton Rancheria reservation and provides the main access to the casino resort.

The latest in a string of legal challenges aimed at slowing or derailing the casino project, the new lawsuit seeks a court order to halt work on the road, which has yet to begin.

The Stop the Casino 101 group, which is named as the plaintiff, says the work would threaten the endangered California tiger salamander. Last August, the federal government designated 47,000 acres of the Santa Rosa Plain as critical habitat, but excluded the 254-acre Graton Rancheria reservation from the protection zone.

City officials in September inked a deal with the Graton Rancheria under which the tribe is to pay the full cost of widening the narrow, two-lane road.

Officials on Thursday said they didn't know enough about the lawsuit to comment but that the agreement was properly vetted against the state's strict environmental laws.

"As part of our due diligence leading up to these agreements, we performed legal review in regards to (the California Environmental Quality Act) and we believe we are in compliance with that," City Manager Gabe Gonzalez said.

City staff and attorneys, in recommending the Wilfred Avenue deal, cited a March agreement between the state and the Graton Rancheria that permits the tribe to run a Las Vegas-style casino, saying it explicitly excluded the site from further environmental studies.

The lawsuit, though, argues that the state-tribe agreement, in making that ruling, relied on an outdated decision by the National Indian Gaming Commission. That 2009 decision was made before the critical habitat for the salamander was defined, rendering it inoperable in this case, the lawsuit argues.

Stop the Casino 101 is also pressing a lawsuit against Gov. Jerry Brown and federal Indian regulatory agencies, challenging the tribe's sovereignty over its reservation.

You can reach Staff Writer Jeremy Hay at 521-5212 or jeremy.hay@pressdemocrat.com or on Twitter @jeremyhay.