U.S. Sens. Brian Schatz and Mazie Hirono are among 13 senators calling for an investigation into the legality of recently uncovered National Security Agency surveillance programs that collect data on millions of Americans.

The senators, in a letter sent Wednesday to the Privacy and Civil Liberties Oversight Board, also are asking the board to determine whether citizens’ privacy was placed at risk and report its findings publicly.

All senators are scheduled to receive a classified briefing on the programs today by officials of the NSA, the Department of Justice, the FBI and the Office of the Director of National Intelligence.

"The Congress has a role in overseeing the administration to make sure that the balance between national security concerns and privacy is struck both under the Constitution and in the law," Schatz said in a telephone interview from Washington, D.C. "I have confidence that the members of the Senate in both parties are going to be aggressive in getting to the bottom of this and seriously considering whether or not the existing law authorized the current activities."

Hirono’s office said she was attending classified Armed Services Committee briefings on Wednesday and was not immediately available for comment.

The Privacy and Civil Liberties Oversight Board includes five members appointed by the president and confirmed by the Senate. It oversees executive branch agencies, including the Department of Justice, and agencies under its jurisdiction such the FBI and the Department of Homeland Security.

In their letter, senators detail concerns over the programs unveiled by media last week. One program gathered hundreds of millions of U.S. cellphone records; the other, known as Prism, collected from nine U.S. tech companies Internet usage data on foreign users.

The letter asks the oversight board to make it an urgent priority to determine whether the programs are "conducted within the statutory authority granted by Congress" and to "take the necessary precautions to protect the privacy and civil liberties of American citizens under the Constitution."

"The Privacy and Civil Liberties Oversight Board was established under the law for situations such as these," Schatz said. "It’s important to understand the scope of the program and how it fits under the existing statutory framework."

The programs reportedly are authorized under provisions of the Foreign Intelligence Surveillance Act and amendments approved in 2008 and 2012.

Schatz, with his first vote in December as a newly appointed senator, voted against the reauthorization of the 2008 amendments. As a member of the House in 2012, Hirono did not cast a vote on the reauthorization bill. She had voted against the amendments in 2008.

Members of the Senate Intelligence Committee were briefed Tuesday. House members were invited to a similar briefing Tuesday.