Lawmakers are demanding that the Obama administration comes clean on how many Americans have been caught up in its domestic surveillance programs.

Without that information, a bipartisan group of leading lawmakers aren't able to fully determine what changes need to be made to US surveillance laws, some of which will expire by the end of 2017.

"You have willingly shared information with us about the important and actionable intelligence obtained under these surveillance programs," wrote a letter to director of national intelligence James Clapper, on Friday. "Now we require your assistance in making a determination that the privacy protections in place are functioning as designed."

Those surveillance programs include the controversial PRISM program, which reportedly tapped data from nine tech titans including Apple, Facebook, Google, Microsoft and others.

These programs were authorized under Section 702 of the Foreign Intelligence Surveillance Act, a statute that allowed the government to secretly collect any data on foreign citizens, but also collect data on Americans who are incidentally collected as part of that effort.

Critics argue that this is a "backdoor search" on Americans that doesn't require a warrant, contravening Fourth Amendment protections against unwarranted domestic surveillance.

The letter asked "simply for a rough estimate" of how many Americans were ensnared in the programs, rather than an "exact count." Clapper has never publicly said how many searches, or how many people ordinarily protected under the constitution, were affected.

Now Congress wants to know before it reauthorizes any of the provisions that Section 702 permit.

But that might be a tough task, given how lawmakers have faced Clapper's "reluctance to provide this information in the past."

Sen. Ron Wyden (D-OR), a civil liberties and privacy advocate, has been asking for this information since 2011, when he first revealed that there was a "secret" interpretation of the Patriot Act. Thanks to disclosures by whistleblower Edward Snowden, that interpretation allowed the government to collect any "tangible" data from a company, like calling records from a phone company.

In a letter to Wyden in 2014, Clapper only went as far as to say that that there "have been queries, using US person identifiers, of communications lawfully acquired to obtain foreign intelligence targeting non-US persons reasonably believed to be located outside the United States."

Clapper has a choice of handing over that unknown figure of Americans affected and take a short-term publicity hit, or deny the House members' request and face a watering down or killing of the statute.

But considering that the statute is only used to collect a fraction of all the National Security Agency's intelligence, the lawmakers' efforts might be a little displaced.

Former NSA analyst turned whisleblower William Binney said a little-known executive order is a "blank check" for the intelligence community to use when all other laws fail, or simply don't reach far enough.

According to the NSA, the executive order -- its contents and interpretations of which are classified -- grants the agency the majority of its authority.

The House members gave Clapper until May 6 to respond.