The director of the National Security Agency, under fire for his department’s widespread phone and Internet surveillance programs, told lawmakers Wednesday that such data collections have foiled “dozens” of terrorist events.

Addressing the simmering controversy in the first open congressional hearing on the matter, Army Gen. Keith Alexander, NSA’s director and head of U.S. Cyber Command, defended the massive collection of Americans’ phone records, saying such practices were essential to protecting the homeland.

Alexander said in a Senate Appropriations Committee hearing Wednesday that those programs had foiled “dozens” of terrorists plots but was unable to provide a specific figure. He told lawmakers he would give them an exact number later in the week.

Lawmakers also focused on Edward Snowden, the former CIA employee and NSA contractor who admitted to leaking information about Verizon turning over the phone records of millions of Americans to the federal government — and the accessing of U.S. Internet data.

Sen. Dick Durbin, D-Ill., noted Snowden’s “limited education and work experience” and asked if Alexander was troubled by his access to classified information.

“I am,” Alexander replied.

Intelligence officials have given lawmakers closed-door briefings on the NSA programs throughout the week and will hold another classified meeting later Wednesday.

Lawmakers have complained about being left in the dark on the clandestine programs, an assertion the White House dismissed on Wednesday.

“I think it’s been amply demonstrated that with regards to both sections of the Patriot Act and the programs that exist under those authorities that members of Congress were briefed or had the opportunity to be briefed on them,” White House Press Secretary Jay Carney said. “It is certainly the case that some members of Congress did not avail themselves of the opportunity to be briefed, but that’s certainly their prerogative.”