The National Security Agency (NSA) plans to release details of terrorist attacks thwarted by its controversial bulk surveillance of Americans’ communications data, a senior US senator said on Thursday.

Senator Dianne Feinstein (Democrat, California), the chairwoman of the Senate intelligence committee, said the NSA director, General Keith Alexander, would provide “the cases where this [surveillance] has stopped a terrorist attack, both here and in other places” as early as Monday.

The claim that the surveillance programs helped stop terrorist attacks has come under criticism from two US senators who sit on the intelligence committee.

“When you're talking about important liberties that the American people feel strongly about, and you want to have an intelligence program, you've got to make a case for why it provides unique value to the [intelligence] community atop what they can already have," Senator Ron Wyden, an Oregon Democrat, told the Guardian in an interview on Thursday.

But the FBI director, Robert Mueller, forcefully defended the programs on Thursday to the House judiciary committee by saying the broad surveillance could have foiled the 9/11 attacks and averted “another Boston”.

Feinstein’s comments followed an afternoon briefing attended by 47 senators about two NSA programs recently disclosed by the Guardian: one that collects the phone records of millions of Americans; and another, known as Prism, that targets the online communications of individuals believed to be outside the US. For many senators, it was their first exposure to the details of how the programs operate.

Yet the programs may soon change. Feinstein said she had “tasked director [of national intelligence James] Clapper to consider the program, to present some changes, if he feels it necessary. We will consider changes.”

She added: “We will certainly have legislation which will limit or prevent contractors from handling highly classified technical data.”

The Los Angeles Times reported that Edward Snowden, a former Booz Allen Hamilton contractor to the NSA, used a thumb drive to exfiltrate data about the surveillance programs to the Guardian and the Washington Post.

Feinstein also cleared up a lingering uncertainty about the role of the courts in overseeing the NSA’s ability to comb through its database of the phone records of millions of Americans. The NSA has the ability to search the database unilaterally.

“To search the database you have to have reasonable, articulable cause to believe that an individual is connected to a terrorist group,” Feinstein said. “Then you can get the numbers. If you want to collect content, then you get a court order.”

Pressed by the Guardian if that meant the NSA did not require a court order to search through the database, she replied, “That’s my understanding.”

In a heated Senate appropriations committee hearing on Wednesday, the NSA chief, General Alexander, said: “We don't get to swim through the data,” and that searching through it requires a “very deliberate process.” But that process is not overseen by a judge ahead of time, according to the Senate intelligence committee chairwoman.

Feinstein also said that before any content could be searched pursuant to a court order, all the NSA possesses is “the name and the number called, whether it’s one number or two”.

Yet US intelligence leaders have firmly denied its phone-records databases contain any names of any subscribers.

“The information acquired does not include the content of any communications or the identity of any subscriber,” according to a 6 June factsheet released by Clapper. It is unclear if Feinstein misspoke or learned new information at the briefing, as she spoke to reporters for about four minutes before leaving to catch a plane.