Jacquelyn Martin, Associated Press
Uniformed Secret Service agents patrol the top of the White House as seen from the South Lawn of the White House in Washington, Tuesday, March 17, 2015. According to the Secret Service a letter sent to the White House tentatively tested positive for cyanide. The letter was received at an off-site mail screening facility on March 16 and additional testing Tuesday returned a "presumptive positive" for cyanide.

WASHINGTON — The chairman of the House Oversight Committee says he is issuing subpoenas to two Secret Service agents who witnessed an episode in which two high-ranking agency officials are accused of driving into a secure area at the White House without authorization.

Republican Rep. Jason Chaffetz of Utah says the agents can shed light not only on the March 4 incident, "but also on why the Secret Service appears to be systemically broken and in desperate need of both leadership and reform."

Chaffetz had asked Secret Service Director Joseph Clancy to allow four agents and officers to testify at a hearing last week on the March 4 incident. Clancy declined the request and instead testified as the sole witness.