WASHINGTON (AP) _ Nineteen bus drivers who transport special education students in the District of Columbia have been fired after school officials discovered some of them have drunken-driving convictions or have tested positive for drug use.

School officials said Wednesday they expect more drivers to be fired in the weeks to come as the FBI completes background checks on the rest of the bus force.

Three administrative clerks also were dismissed for tampering with bus driver records. School spokeswoman Denise Tann would not reveal the identity of the employees, but said the 19 drivers were employed by a contractor hired to oversee bus operations.

District School Superintendent Arlene Ackerman has asked the FBI to conduct background checks on all drivers, which include 200 employed by an outside firm and 100 employed by the school district.

The 19 fired drivers had drunken-driving convictions, had tested positive for drug-use, lacked valid commercial driver's license or had committed other offenses that could be harmful to the safety of the children, officials said.

The district provides bus service primarily for several thousand special education students to schools throughout the area.

Concerns were raised in October when six drivers were dismissed for failing drug tests, and a clerk in the safety and training office was fired for covering up those test results. Last week, school officials ordered bus drivers to undergo immediate drug testing.