Two US senators on the panel overseeing the National Security Agency said intelligence officials were "unable" to demonstrate the value of a secret surveillance program that collected and analyzed the internet habits of Americans.

Senators Ron Wyden (Democrat, Oregon) and Mark Udall (Democrat, Colorado), the chief inquisitors of US intelligence officials during the current surveillance scandal, added a sharp warning late Tuesday that senior intelligence officials "are not always accurate" in their public statements about the scope and utility of their wide-ranging surveillance efforts.

Wyden and Udall were specifically reacting to a program, revealed last week by the Guardian, that gathered and analyzed bulk internet "metadata" records from Americans, such as the subject lines of their email communications and their internet protocol (IP) addresses, detailing where their devices accessed the internet. Senior intelligence officials told the Guardian that the program ended in 2011.

"We were very concerned about this program's impact on Americans' civil liberties and privacy rights, and we spent a significant portion of 2011 pressing intelligence officials to provide evidence of its effectiveness," Wyden and Udall said in a statement late Tuesday, the first senators to acknowledge the internet metadata collection. "They were unable to do so, and the program was shut down that year."

Shawn Turner, the chief spokesman for director of national intelligence James Clapper, who is currently under congressional fire over the truthfulness of his testimony on the surveillance efforts, told the Guardian last week that the Obama administration unilaterally ended the program for "operational and resource reasons".

Wyden and Udall held out the bulk internet metadata program as a cautionary tale about intelligence agencies misrepresenting their surveillance programs – a significant escalation of pressure on US intelligence leaders coming after Clapper disclosed on Tuesday a letter apologizing for "clearly erroneous" statements made to Wyden.

"In our judgment it is also important to note that intelligence agencies made statements to both Congress and the [Fisa] Court that significantly exaggerated this program's effectiveness," Wyden and Udall said. They did not elaborate.

"This experience demonstrates to us that intelligence agencies' assessments of the usefulness of particular collection programs – even significant ones – are not always accurate. This experience has also led us to be skeptical of claims about the value of the bulk phone records collection program in particular."

In 2011, Wyden and Udall began warning their colleagues and reporters that the breadth of US surveillance efforts authorized under the Patriot Act and a 2008 expansion of the Foreign Intelligence Surveillance Act (Fisa) had transgressed the bounds established by the plain text of the laws authorizing them. They were prevented from detailing what they meant by laws preventing the public discussion of classified information.

Since the surveillance revelations disclosed to the Guardian and the Washington Post by whistleblower Edward Snowden, Wyden and Udall have declined to call for the resignation of any intelligence leaders. While they have challenged the NSA's claims that bulk phone records collection had significantly aided US defenses against terrorism, they have occasionally muted their criticisms. "I think details about overseas intelligence collection should be kept secret," Wyden told the Guardian on 9 June.

Tuesday's statement represents their most pointed criticism to date – about both the value of the NSA's surveillance efforts and the integrity of the men and women in charge of them.

"We believe that the broader lesson here is that even though intelligence officials may be well-intentioned, assertions from intelligence agencies about the value and effectiveness of particular programs should not simply be accepted at face value by policymakers or oversight bodies any more than statements about the usefulness of other government programs should be taken at face value when they are made by other government officials," Wyden and Udall said.

"It is up to Congress, the courts and the public to ask the tough questions and press even experienced intelligence officials to back their assertions up with actual evidence, rather than simply deferring to these officials' conclusions without challenging them."

Officials from the NSA and the Office of the Director of National Intelligence had no immediate reaction to Wyden and Udall's statement.

Echoing an intimation made on Friday in a letter to Clapper that was signed by 26 senators, Wyden and Udall hinted that the true breadth of US surveillance on Americans has yet to come out.

"The fact that Patriot Act authorities were used for the bulk collection of email records as well as phone records underscores our concern that this authority could be used to collect other types of records in bulk as well, including information on credit card purchases, medical records, library records, firearm sales records, financial information and a range of other sensitive subjects," the two senators said.

"These other types of collection could clearly have a significant impact on Americans' constitutional rights."