The trio of congressional officials who have exclusive control over traffic rules on the Capitol grounds are ordering a change that would enhance Capitol Police’s authority to search backpacks, suitcases and other personal belongings carried onto the 290-acre property.

Under a section of code banning firearms, dangerous weapons and explosive devices, the language approved by the Capitol Police Board on July 15 states: “At the direction of the Chief of Police, the United States Capitol Police may search packages, bags, and other containers in the immediate possession of individuals who enter and are within the United States Capitol Grounds for the purpose of detecting prohibited items.”

Under current law, Capitol Police need probable cause to conduct such searches when they encounter suspicious individuals on campus. Even with that authority, they did not stop or search Leo P. Thornton on April 11, before he committed suicide on the Lower West Terrace.

The 22-year-old man — who allegedly carried a backpack, suitcase and a gun in his pocket — died of a self-inflicted gunshot wound on an otherwise calm Saturday afternoon. Multiple law enforcement agencies responded to the scene, placing the Capitol and Capitol Visitor Center on lockdown as a Capitol Police bomb squad investigated his belongings.

Although police found no hazardous materials in Thornton’s bags, that incident and the specter of lone-wolf attacks crop up in discussions about the new policy — which legal experts say constitutes an overreach of authority.

In an interview with CQ Roll Call, Senate Sergeant-at-Arms Frank J. Larkin said police would only implement the measure if they had “high-confidence intelligence” about a threat or if the Capitol was being attacked. Larkin emphasized that the authority may never have to be exercised, but he said having it in place would preserve what he characterized as the open, accessible campus and provide for the safety of the legislative branch.

“We are not immune — just because we are on Capitol Hill — we are not immune,” Larkin said. He stressed tough, effective security policing as “the only way we can inoculate ourselves.”

Scenes from Chattanooga, Tenn., where a shooter used an AK-47-style gun to shoot four Marines on July 16, flashed across the TV in Larkin’s Capitol office on the following morning. The suspect in that case has not been linked to international terror groups, but news of coordinated plots and mass shootings have increased concern under the Dome, a magnet for threats.

But the regulation would establish a troubling precedent in the eyes of Tim Lynch, a veteran attorney at the Cato Institute. Lynch predicted courts would “definitely be hearing a challenge” if Capitol Police try to assert search power across the campus, which extends south, roughly, from the plaza in front of Union Station to D Street Southeast and into the city’s Southwest quadrant to the Ford Office Building.

“In some ways, you know, they’ve tried to establish these search powers in certain distinct areas, but the difference is that people are kind of put on notice,” Lynch said.

Tourists approaching the Capitol Visitor Center meet signs listing prohibited items, and they are herded through metal detectors and X-ray machines. Transit authorities warn airline passengers they will submit to search before flight. Even Metro riders see posted warnings.

“If you decline to be searched, you’re going to have to leave,” Lynch said. “And that’s not going to be feasible in these outdoor spaces.”

The officials of the Capitol Police Board — Larkin, House Sergeant-at-Arms Paul D. Irving and Architect of the Capitol Stephen T. Ayers — plan to work with their congressional overseers to develop signs to place around the grounds when the regulation takes effect. (Pursuant to federal law, the amendment must be printed in one of Washington’s daily newspapers 10 days before it takes effect. That date remains unclear.) Signs would appear at the head of walkways, cab stands and near bus drop-offs for maximum visibility.

“Fourth-Amendment-Free Zone,” suggested Arthur B. Spitzer, legal director for the American Civil Liberties Union of the Nation’s Capital, after he reviewed the amendment.

“We’ve all gotten pretty much used to the idea that you have to do that when you’re entering any of the buildings of the Capitol — you go through a metal detector. Same at a court house or city hall,” Spitzer said. “That’s a different thing than just somebody who’s minding his own business, walking across the Capitol grounds. The Capitol grounds are open to the public.”

Would “high-confidence intelligence” apply only to threats specifically targeted to the Capitol? Could the anniversary of the 9/11 terror attacks or a call to arms during Ramadan trigger enhanced search circumstances?

When Canada’s legislature was locked down in October 2014, as police chased a gunman through the main Parliament building, Capitol Police said they were monitoring the situation, but not taking any “significant” steps to heighten security around Congress.

Spitzer also wondered whether the amendment “is really going to be a green light for racial and ethnic profiling.”

Responding on June 11 to a 911 call about a black man carrying a gun in his waistband near the Russell Senate Office Building, Capitol Police briefly stopped a retired officer from another law enforcement agency. After the officer’s credentials were verified, he was released.

Libertarians in Congress, including Kentucky Republican Rep. Thomas Massie, said they would be leery of warrantless searches of people’s property. Rank-and-file members, who did not want to be identified talking about a policy they had not yet seen, also called it worrisome.

Senate Rules and Administration Chairman Roy Blunt, R-Mo., told CQ Roll Call he had an opinion on the amendment, but was not ready to share it. His counterpart in the House, Michigan Republican Candice S. Miller, expressed her support for the change.

“The safety and security of the U.S. Capitol grounds is paramount. The Capitol Police Board is taking necessary steps to respond to the threats we unfortunately face. Searches are a part of our daily lives, we encounter them at the airport, when entering federal buildings, and courthouses. These administrative searches will help us fulfill our responsibility to keep our constituents safe when they visit this icon of American democracy,” Miller, the chairwoman of the House Administration Committee, said in a statement for this story.

In June 2014, the Capitol Police Board rolled out the the first major amendment to the Capitol traffic regulations in more than 30 years, following the exhaustive work of a task force of law enforcement officials. That overhaul of campus traffic rules sparked anger within Washington’s cycling community in particular. Backlash against this change could land in federal court.

Spitzer thinks it should be stopped. “Otherwise, they’re going to be exposing their Capitol Police employees to successful lawsuits for damages for violating people’s Fourth Amendment rights,” he said. “And really that shouldn’t be their main concern. Their main concern should be, they don’t want to violate people’s constitutional amendment rights.”

Correction 12:56 a.m.

A previous version of this article misquoted a section of code banning firearms, dangerous weapons and explosive devices.