After military police detained two journalists last month outside a military manufacturing plant, an Army spokesman said the journalists had violated “Federal law and Army Regulations” by photographing the facility.

But which law, and which regulations? The Army didn’t say at the time—and it won’t say now.

We have a pretty good idea, though, of one federal law the government is likely to cite when it responds to a lawsuit filed by The Blade of Toledo, OH. And if the case goes to trial, it could present the first constitutional challenge to a statute that’s been on the books since the World War II era.

All of this stems from a March 28 incident at the Joint Systems Manufacturing Center (JSMC) in Lima, OH, where Blade photographer Jetta Fraser reportedly took photos of the center while standing in a small roadway between the public street and the guard hut at the facility’s entrance. When she tried to leave, with Blade reporter Tyrel Linkhorn, military police detained and questioned them—and confiscated their cameras and deleted digital photos, according to a report in The Blade. Notably, the journalists have not been charged with trespassing.

The newspaper filed a federal lawsuit April 4 against various government officials alleging, among other things, that military police had interfered with the journalists’ lawful exercise of their First Amendment rights. Before and after the lawsuit was filed, Don Jarosz, deputy public affairs officer for the Army’s TACOM Life Cycle Management Command, released a statement asserting the journalists had “taken unauthorized photographs of the installation” from “within the boundaries” of the facility and noting:

JSMC Lima is a restricted Department of Defense Government-owned, Contractor-operated facility that fabricates and assembles armored combat vehicles and equipment for U.S. and Foreign Military customers. According to Federal law and Army Regulations, it is unlawful to take any photograph without first obtaining permission of the commanding officer. Signage to this effect is visible and warns that any such material found in the possession of unauthorized personnel will be confiscated.

In response to inquiries from CJR, Jarosz refused last week to say which “Federal law and Army Regulations” were applicable, adding that it is “standard Army policy not to comment on any pending litigation.” Never mind that I did not ask Jarosz to comment on pending litigation—he was asked to clarify a reference in the statement he released both before and after the lawsuit was filed.

But while it remains unclear which “Army Regulations” are at issue, a close reading of the statement, informed by discussions with half a dozen legal scholars, revealed that one federal law is very likely to be used by the government in its response to The Blade’s lawsuit: 18 U.S.C. § 795, titled “Photographing and sketching defense installations.” It states:

Whenever, in the interests of national defense, the President defines certain vital military and naval installations or equipment as requiring protection against the general dissemination of information relative thereto, it shall be unlawful to make any photograph, sketch, picture, drawing, map, or graphical representation of such vital military and naval installations or equipment without first obtaining permission of the commanding officer of the military or naval post, camp, or station, or naval vessels, military and naval aircraft, and any separate military or naval command concerned, or higher authority, and promptly submitting the product obtained to such commanding officer or higher authority for censorship or such other action as he may deem necessary.

Penalties include a fine and up to one year in prison.

The government could use that statute to argue that military police had good reason to detain the Blade journalists, on the theory that Fraser, “without first obtaining permission of the commanding officer,” unlawfully photographed a “military installation” defined by the president as “vital” and “requiring protection against the general dissemination of information relative thereto.” Embedded in the government’s argument would be Executive Order 10104, issued in 1950 to define the military installations requiring protection. It includes those that are designated as “restricted,” and Jarosz’s statement calls JSMC, which manufactures combat vehicles and defense systems, “a restricted Department of Defense Government-owned, Contractor-operated facility” (emphasis added).

Jonathan Peters is CJR's press freedom correspondent. An attorney, he is an assistant professor of journalism at the University of Kansas, where he teaches and researches media law and policy, with an affiliate research position exploring big data and Internet governance in the KU Information & Telecommunication Technology Center. Peters has blogged on free expression for the Harvard Law & Policy Review, and he has written on legal issues for Esquire, The Atlantic, Slate, The Nation, Wired, and PBS. Follow him on Twitter @jonathanwpeters.