This photo taken March 12, 2013, shows Director of National Intelligence James Clapper testifying before the Senate Intelligence Committee. (Susan Walsh/AP)

A bipartisan group of 26 senators is pressing the director of national intelligence to shed more light on the scope of a controversial surveillance program to collect the phone records of tens of millions of Americans with no connection to wrongdoing.

In a letter Friday, the group, led by Sen. Ron Wyden (D-Ore.), said that the bulk collection program under Section 215 of the Patriot Act “raises serious civil liberties concerns and all but removes the public from an informed national security and civil liberty debate.”

The current program, whose existence and evolution was revealed in recent weeks following disclosures of top-secret documents obtained by The Washington Post and the British newspaper the Guardian, was authorized by a federal surveillance court in 2006 under an interpretation of the law that remains secret. Civil liberties groups and lawmakers have pressed for declassification of the court opinion.

“We are concerned that by depending on secret interpretations of the Patriot Act that differed from an intuitive reading of the statute, this program essentially relied for years on a secret body of law,” the senators wrote.

The program collects metadata — phone numbers dialed, the duration of the phone calls and device numbers. It does not intercept the calls’ content or gather subscribers’ names and addresses.

The senators are pressing Director of National Intelligence James R. Clapper Jr. to answer a series of questions, including how long the National Security Agency has used Section 215 authorities to engage in the bulk collection of Americans’ records.

A classified NSA draft inspector general report obtained by The Post and the Guardian stated that bulk collection of phone records, outside of court or congressional authorization, began in the fall of 2001, shortly after the terrorist attacks of Sept. 11. But the government has not declassified that fact.

The senators also want to know whether Section 215 authorities have been used for the bulk collection of records pertaining to Americans beyond phones, as well as whether the NSA has collected or made any plans to collect Americans’ cell-site location data in bulk.

Intelligence officials have said that the bulk phone collection program under Section 215 does not collect location data. But they have not said whether other programs do so.

In their letter, the senators also asked whether there have been any violations of the court orders permitting the bulk phone records collection.

The senators also pressed Clapper to specify instances in which intelligence gained by reviewing phone records from the program proved useful in thwarting a particular terrorist plot, and instances in which useful intelligence was gained by reviewing phone records that could not have been obtained without the bulk collection authority.