Environmentalists have filed a lawsuit aimed at stopping construction of the Interstate 69 extension in southern Indiana, claiming the project violates federal environmental laws.

Landowners affected by the I-69 project, Citizens for Appropriate Rural Roads and the I-69 Accountability Project filed the suit in federal court in Indianapolis on Monday, a week after federal highway officials signed off on the final environmental review for the stretch from the Crane Naval Surface Warfare Center to Bloomington.

The groups contend that the highway project threatens the endangered Indiana bat and violates the federal Endangered Species Act.

A spokesman for Citizens for Appropriate Rural Roads said the group believes the state highway department underestimated in its count of nearly 1,500 geologic features such as sinkholes, caves and springs in the path of the latest approved section.

"I-69 should never have been routed through this extremely environmentally sensitive area," Thomas Tokarski said.

Indiana Department of Transportation spokeswoman Cher Elliott told The Herald-Times of Bloomington that the agency has "followed all state and federal guidelines" regarding the project.

Construction began in 2009 near Evansville on the first section of the planned 142-mile route between that city and Indianapolis. The project has been estimated to cost about $3 billion to complete.

In February, the Hoosier Environmental Council and Citizens for Appropriate Rural Roads filed a lawsuit claiming the Army Corps of Engineers violated federal law by giving the state permission to fill wetlands and reroute streams along the I-69 route in Greene and Daviess counties.

That lawsuit, which is still pending in federal court, asked that the highway be rerouted to a less environmentally disruptive route along existing highways, U.S. 41 and Interstate 70.