Senators are questioning whether the National Security Agency collected bulk data on more than just Americans’ phone records, such as firearm and book purchases.

A bipartisan group of 26 senators, led by Sen. Ron Wyden (D., Ore.) asked Director of National Intelligence James Clapper to detail the scope and limits of the National Security Agency’s surveillance activities in a letter released Friday.

"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 in the letter.

The NSA’s surveillance program has come under intense scrutiny following a leak revealing the agency harvested the phone metadata of millions of American citizens.

The senators noted that the federal government’s authority under Section 215 of the PATRIOT Act is broad and rife with potential for abuse. Among the senators’ concerns was whether the NSA’s bulk data harvesting program could be used to construct a gun registry or violate other privacy laws.

"It can be used to collect information on credit card purchases, pharmacy records, library records, firearm sales records, financial information, and a range of other sensitive subjects," the senators wrote. "And the bulk collection authority could potentially be used to supersede bans on maintaining gun owner databases, or laws protecting the privacy of medical records, financial records, and records of book and movie purchases."

The senators asked Clapper in the letter whether the NSA used PATRIOT Act authorities to conduct bulk collection of other types of records, and whether there are any instances of the agency violating a court order in the process of such collections.

Civil libertarians say such surveillance is a violation of privacy. However, the government has defended the program, saying it helped thwart several terrorist attacks and is minimally invasive.

Second Amendment groups and Republican members of Congress have long warned against the creation of a national gun registry. Fears of such a registry bogged down several attempts to forge a bipartisan gun-control bill in the Senate earlier this year.

"In this country, the government can't just monitor your constitutionally

protected activities—like gun ownership—just because it wants to," said Brian Phillips, a spokesman for Sen. Mike Lee (R., Utah), who signed onto the letter. "The justification that, ‘if you're not doing anything wrong, you don't have to worry about it,' turns us into a police state very quickly. That's why

Congress is right to seek broad oversight of the NSA's data collection programs."