WASHINGTON — The head of the Transportation Security Administration vowed Tuesday to take long-overdue steps to improve security at the hundreds of overseas repair shops that work on U.S. airplanes.

Facing aggressive questioning, TSA Administrator Kip Hawley told a Senate hearing that the agency is developing security rules that repair facilities will have to meet by early 2009.

Some lawmakers say the repair shops, where airplanes undergo maintenance and overhauls, have shoddy security that makes them vulnerable to terrorists who could plant a bomb inside an airplane.

The TSA itself wrote in 2005 that "there is a critical need to establish security requirements" for the shops, according to a TSA draft of proposed rules that was never enacted. "Repair organizations that do not employ any type of security measures may provide an attractive target for terrorist organizations attempting to evade aviation security protections currently in place," the TSA wrote.

Sen. Claire McCaskill, D-Mo., told Hawley that neglecting security for repair shops "is a disaster waiting to happen." She criticized the TSA for missing an August 2004 deadline set by Congress to write the security rules and a February 2006 deadline to inspect the 700 overseas repair shops that work on U.S. airplanes.

A law requiring security rules was enacted in December 2003 as lawmakers worried that U.S. airlines were having more planes repaired at the 700 U.S.-licensed overseas shops. Most are in Europe and Asia. Some are in Saudi Arabia, Egypt and Jordan. The shops operate independently of airlines.

Hawley, TSA chief since July 2005, did not explain the missed deadlines or set a timetable for when the rules would be finalized. He said that the agency will meet Congress' new deadline to have the rules written by August and that other measures now protect against bombs being planted in airplanes at repair shops.