An 804-mile-long transmission line in the Midwest appeared to be finally ready for construction after more than a decade of delays.

Known as Grain Belt Express, the $11 billion line would carry electricity produced by wind farms in Kansas across Missouri and Illinois all the way to Indiana, making it the very sort of infrastructure that experts say is needed to modernize America’s aging electrical grid.

But on Tuesday, the Missouri attorney general, Andrew Bailey, a Republican, opened an investigation into Grain Belt Express and requested that the state’s Public Service Commission reconsider its approval.

The development could be a major setback for Invenergy, the energy company that is behind the project, and its founder Michael Polsky, a billionaire immigrant who has been working to sell the public on Grain Belt Express. The company calls it the largest privately funded transmission line in the country’s history.

Mr. Bailey claims that developers of the transmission line, which could carry enough energy to power more than 3 million homes, fraudulently inflated the number of jobs it would create, overstated cost savings to consumers, and misled landowners. He said his intention was “ultimately killing this project.”

Mr. Bailey’s investigation will be welcome news to other opponents of the project, which include Senator Josh Hawley, Republican of Missouri, some trade groups and landowners who don’t want high voltage power lines to run through their property. Last week Senator Hawley urged the Energy Department to cancel a conditional loan guarantee it has offered Grain Belt Express.