US Transportation Security Administration (TSA) head John Pistole will retire effective Dec. 31, TSA announced Thursday.

Pistole, formerly the number two official at the Federal Bureau of Investigation, has been the TSA administrator since June 2010, overseeing a sprawling agency with 60,000 workers and the responsibility for security at more than 450 US airports. “It has been an honor and a privilege to have served as TSA administrator,” Pistole said in a brief statement. “No words can convey my deep gratitude for the hard work and dedication of the thousands of men and women committed to protecting the American public.”

During his tenure, Pistole worked to shift the focus of TSA from treating all airline passengers as equal risks to using intelligence and risk analysis when screening passengers. “John Pistole has been integral in leading TSA’s transformation to a risk-based, intelligence-driven counterterrorism agency dedicated to protecting our transportation systems,” US Homeland Security secretary Jeh Johnson said Thursday.

Pistole implemented the Pre-Check expedited screening program. “It helps us do our job” to segment out large numbers of “known” passengers who give information to TSA, enabling the agency to spend more time screening “unknown” travelers at airport checkpoints, Pistole explained last year. “It allows us to be more precise and tailored in our approach,” he said.

Pistole did face resistance to his “risk-based” approach, most notably when he unsuccessfully tried to rescind TSA’s prohibition on passengers carrying items such as pocket knives through airport security checkpoints.