Congressional watchdogs say a half-billion dollar math error may have killed off a massive clean-energy project in Illinois.

The federal government canceled the FutureGen clean-coal project last year, saying the cost had nearly doubled. But the Government Accountability Office told members of Congress today that the inflated numbers were way off. Representative Donna Edwards of Maryland called the error “unbelievable.”

EDWARDS: We should, you know, send everybody to second grade.

The GAO reports that the Mattoon, Illinois-based project would cost about $1.3 billion, not $1.8 billion, as the feds calculated last year. Angela Griffin heads up Coles Together, which promotes economic development in the Mattoon area.

GRIFFIN: We from the very beginning had been jumping up and down saying this is wrong. So it’s a relief that the Government Accountability Office has come to that conclusion. It’s unfortunate that we’ve lost over a year in the life of this project.

Energy Secretary Steven Chu told reporters last week that the government is giving FutureGen a fresh look.