(AP) — John Pistole announced Thursday he is stepping down as chief of the Transportation Security Administration after 4 1/2 turbulent years in which the agency sought to balance passenger privacy against safety and stay a step ahead of terrorists looking for cunning new ways to smuggle bombs aboard planes.

Pistole said he will leave office at the end of the year and expects to accept a position in academia next year. Shortly after his announcement, Anderson University posted a statement on its website that said a search committee will propose to the school's board of trustees that Pistole be the school's next president. The university is located in Anderson, Indiana, Pistole's hometown.

Pistole received a bachelor's degree in 1978 from the nearly 100-year-old university, which was founded by the Church of God, according to the school's statement.

Pistole's retirement comes at a time when the agency is facing one of its toughest challenges since its creation after the Sept. 11, 2001 terrorist attack — how to prevent a terrorist from boarding a plane with a bomb that contains no metal parts and isn't detectable through normal screening. Pistole warned last month that an al-Qaida cell in Syria known as the Khorasan Group has been researching and testing improvised explosive devices designed to elude airport security and represents "a clear and present danger" to commercial flights to Europe and the United States.

Pistole, TSA's administrator for four and a half years, instituted a "risk-based" security philosophy that has enabled the agency to move passengers deemed to be low risk through airport screening more quickly and significantly reduced passenger complaints. The Trusted Traveler and PreCheck programs are used by 5 million passengers per week at 120 U.S. airports, according to the agency.

"John Pistole has been integral in leading TSA's transformation to a risk-based, intelligence-driven counterterrorism agency dedicated to protecting our transportation systems," said Jeh Johnson, secretary of the Department of Homeland Security. "Because of his efforts over the past four and a half years, our country's transportation systems are more safe and secure."

But privacy advocates have complained that the programs put personal information about millions of travelers in the hands of the government and government contractors.

Pistole was nominated by President Barack Obama in 2010. Prior to that, he had spent 26 years with the FBI, most recently as deputy director.

TSA has a workforce of over 60,000 employees and security operations at more than 450 airports throughout the United States.

Pistole ran afoul of flight attendants, airlines and Congress last year when he announced his intention to allow passengers to carry small knives, sports equipment like bats and golf clubs, and other previously prohibited items onto planes. Pistole said the odds of the items being used to threaten the safety of a plane were extremely low, but critics said they worried unruly passengers might use the items as weapons. Pistole was eventually forced to back down.