MIAMI (Reuters) - The largest operator of online schools in the United States is being investigated by the Florida Department of Education over allegations the company may employ teachers who are not properly certified, a state official said on Tuesday.

K12, a company founded by former U.S. Education Secretary William Bennett, is one of a dozen for-profit companies operating full-time, online public elementary and high schools across the country.

The probe was initiated after Florida's Seminole County School District raised questions over whether K12 is using uncertified teachers in violation of state law.

Jamie Mongiovi, an education department spokeswoman, confirmed the investigation, which was first reported by the Florida Center for Investigative Reporting/State Impact Florida.

"However, it is not completed and a report has not been written," Mongiovi said.

According to the report by the Florida Center for Investigative Reporting, state officials are also looking into allegations K12 may have asked employees to cover up any evidence it used uncertified teachers.

In a statement, K12 said it was cooperating with the investigation. The company said it had conducted an internal review and found no evidence of any effort to avoid Florida's teacher certification requirements.

"K12 does not believe the allegations made by Seminole County School District are accurate," it said. "K12 teachers assigned to teach students in Florida are state certified."

In online schools, students work from their homes on computers and follow classes broadcast on the Internet.

The schools are part of a growing push to experiment with cutting-edge technology in education but have proved controversial because they draw money from state education budgets at a time when many of those budgets are being slashed.

The schools have caught the attention of public school administrators grappling with budgetary pressures. However, some critics say kids enrolled full-time in online schools score lower on standardized tests.

Citing documents being examined in the investigation, the report by the Florida Center for Investigative Reporting said K12 asked Seminole County Public Schools in 2009 if it was allowed to use uncertified teachers for some online classes.

The school district consulted with the Florida Department of Education, which responded no, citing state law that requires certified teachers, the report said.

But emails from a K12 employee show some teachers may have signed off on papers saying they taught students whom they never had in their virtual classes - students who may actually have been taught by other teachers who lacked certification, the report said.

K12 operates online public schools in over 30 states, with an enrollment estimated at more than 200,000 students in the United States.

The company is involved in more than 40 school districts in Florida, including Miami-Dade and Broward, two of the largest in the United States.

Shares of K12 on the New York Stock Exchange closed down 3.15 percent on Tuesday.

(Reporting by Kevin Gray; Additional reporting by David Adams; Editing by Cynthia Osterman)