Maryland- Two uniformed men strolled into the main room of the Little Falls library in Bethesda one day last week and demanded the attention of all patrons using the computers. Then they made their announcement: The viewing of Internet pornography was forbidden.

The men looked stern and wore baseball caps emblazoned with the words “Homeland Security.” The bizarre scene unfolded Feb. 9, leaving some residents confused and forcing county officials to explain how employees assigned to protect county buildings against terrorists came to see it as their job to police the viewing of pornography.

After the two men made their announcement, one of them challenged an Internet user’s choice of viewing material and asked him to step outside, according to a witness. A librarian intervened, and the two men went into the library’s work area to discuss the matter. A police officer arrived. In the end, no one had to step outside except the uniformed men.

They were officers of the security division of Montgomery County’s Homeland Security Department, an unarmed force that patrols about 300 county buildings — but is not responsible for enforcing obscenity laws.

In the post-9/11 era, even suburban counties have homeland security departments. Montgomery County will not specify how many officers are in the department’s security division, citing security reasons. Its annual budget, including salaries, is $3.6 million.

Later that afternoon, Montgomery County’s chief administrative officer, Bruce Romer, issued a statement calling the incident “unfortunate” and “regrettable” — two words that bureaucrats often deploy when things have gone awry. He said the officers had been reassigned to other duties.

Romer said the officers believed they were enforcing the county’s sexual harassment policy but “overstepped their authority” and had to be reminded that Montgomery “supports the rights of patrons to view the materials of their choice.”

The sexual harassment policy forbids the “display of offensive or obscene printed or visual material.” But in a library, which is both a public arena and a county workplace, the U.S. Constitution trumps Montgomery’s rules.

At most public libraries in the Washington area, an adult can view pornography on a library computer more or less unfettered. Montgomery asks customers to be considerate of others when viewing Web sites. If others are put off, librarians will provide the viewer of the offending material with a “privacy screen.”

Fairfax County forbids library use of the Internet to view child pornography or obscene materials or to engage in gambling or fraud. But Fairfax library spokeswoman Lois Kirkpatrick said, “Librarians are not legally empowered to determine obscenity.”

D.C. library spokeswoman Monica Lewis said the system is working on guidelines for Internet use, but she added that recessed computer screens generally ensure patrons their privacy.

Although many library systems in the United States use filtering software, the D.C. and Fairfax systems do not, and Montgomery uses such software only on computers available to children. Leslie Burger, president-elect of the American Library Association, said the reality is that “libraries are not the hotbed of looking at porn sites.”

Still, Montgomery plans to train its homeland security officers “so they fully understand library policy and its consistency with residents’ First Amendment rights under the U.S. Constitution,” Romer said in his statement.