ATLANTA - Georgia education officials on Thursday ordered investigations at 191 schools across the state where they found evidence of tampering on answer sheets for the state's standardized achievement test.

The order came after an inquiry on cheating by the Governor's Office of Student Achievement raised red flags regarding one in five of Georgia's 1,857 public elementary and middle schools. A large proportion of the schools were in Atlanta.

The inquiry flagged any school that had an abnormal number of erasures on answer sheets where the answers were changed from wrong to right, suggesting deliberate interference by teachers, principals or other administrators.

Experts said it could become one of the largest cheating scandals in the era of widespread standardized testing.

"This is the biggest erasure problem I've ever seen," said Gregory Cizek, a testing expert at the University of North Carolina. "This doesn't suggest that it was just kids randomly changing their answers; it suggests a pattern of unethical behavior on the part of either kids or educators."

Cizek praised Georgia for conducting the analysis, saying that many states do not monitor erasure rates to check for potential cheating.

Kathleen Mathers, the executive director of the state Office of Student Achievement, which is separate from the state Department of Education and is controlled by Gov. Sonny Perdue, said the statewide analysis was done after a smaller one -- prompted in part by articles in the Atlanta Journal-Constitution -- led to an inquiry that found cheating at four schools. "It's an extra step to take to make sure the data accurately represents what our kids have learned, so that we know the best way to help them," Mathers said.

The Georgia analysis looked at 2009 test results from more than 125,000 students taking the state's Criterion-Referenced Competency Test, given in first through eighth grades in reading, language and math. The test is used primarily to measure school progress under the federal No Child Left Behind law. Students in some grades must pass the test before they can be promoted.