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(The following article by Sewell Chan and Kareem Fahim was posted on the New York Times website on July 24.)

NEW YORK — Even more than security experts and intelligence analysts, one group of employees has become central to the new program of bag searches in the transit networks of the New York region: lawyers.

The decision last week to have police officers inspect the belongings of thousands of subway riders has opened a thicket of legal and constitutional issues, involving criminal procedure, transit security and concerns about potential misuse of the new tactic.

Yesterday, Donna Lieberman, the executive director of the New York Civil Liberties Union, said the organization had begun work on a federal lawsuit, which could be filed this week. Such a challenge will most likely claim that the policy violates the Fourth Amendment’s prohibition against “unreasonable searches and seizures.”

And at a news conference in Brooklyn, Capt. Eric Adams, the president of a group of black police officers, said its members were worried that riders of Middle Eastern, African or Asian descent would be disproportionately targeted in the searches, despite official assurances to the contrary.

Tomorrow, New Jersey Transit and the Port Authority of New York and New Jersey plan to start random searches on their trains and in their buses and stations, joining the city police and the Metropolitan Transportation Authority, which began conducting searches on Friday.

The four agencies have tried to sidestep a potential legal minefield by carefully specifying the limits and objectives of the policies, but the legality of the searches could well rest on subtle distinctions in the way they are carried out.

“It is by no means a foregone conclusion that these searches will be found reasonable under the Fourth Amendment,” said Tracey Maclin, a law professor at Boston University. “The number of people involved, the nature and severity of the intrusions and the uncertain duration of the searches all make this is fundamentally very different from anything we have seen before.”

At least three United States Supreme Court cases will probably influence any judge assessing the searches.

In 1979, in Delaware v. Prouse, the court held that random traffic stops, left to the discretion of police officers, were unconstitutional. In 1990, in Michigan v. Sitz, the court upheld the legality of sobriety checkpoints in which a consistent proportion of drivers was stopped. The court ruled that such roadblocks were permitted as a way to prevent drunken driving.

The court has set limits on those roadblocks, however. In 2000, in Indianapolis v. Edmond, it struck down the use of traffic checkpoints to stop drivers so that trained dogs could sniff the vehicles for narcotics. Unlike checkpoints to make sure that drivers were sober or had valid licenses and registrations, the court said, roadblocks for general law enforcement were unconstitutional.

David D. Cole, a law professor at Georgetown University, said the government was likely to succeed in demonstrating a special need for the current searches in New York.

A judge evaluating a legal challenge, he said, would probably look at three factors: whether the searches are truly random or use standardized criteria; whether riders were given advance notice or provided their consent to the searches; and the degree of intrusiveness. “It’s very hard to predict how these cases will turn out, because it’s such an open-ended balancing test,” he said.

According to a two-page directive sent to all police commanders late Thursday, the searches were begun “to increase deterrence and detection of potential terrorist activity and to give greater protection to the mass transit-riding public.” The directive authorized the police to inspect “backpacks, containers and other carry-on items which are capable of containing explosive devices.”

The policy includes several provisions that could help in defending the new searches from court challenges.

First, although Mayor Michael R. Bloomberg and Police Commissioner Raymond W. Kelly have described the searches as “random,” they should rely on a precise frequency that is not subject to officers’ discretion, according to the directive.

A supervisor at each checkpoint is supposed to determine the frequency of searches – 1 in every 5, 12 or 20 passengers, for example – based on the volume of passengers, the number of officers available and the “flow of commuter traffic.”

Second, extensive steps were taken to notify the public about the searches. At subway stations and train terminals, megaphone and public-address systems were used. Notices were handwritten on dry-erase boards in the booths in most subway stations. At many checkpoints, the police have set up signs on easels near the turnstiles.

The directive states that individuals who refuse to be searched can leave the subway system, and that such a refusal “shall not constitute probable cause for an arrest or reasonable suspicion for a forcible stop.”

The degree of intrusiveness could become a thorny issue. The directive provides that officers may open a package and “physically inspect and manipulate the contents to ensure it does not contain an explosive device.” Leaving aside the issue of illegal drugs or weapons, some riders have already voiced misgivings about having the police examine sensitive possessions, like medications and personal hygiene products.

Lawyers have been used extensively in writing the search policies. The deputy police commissioner for legal matters, S. Andrew Schaffer, formerly the general counsel for New York University, joined Mr. Kelly and top police commanders on Thursday morning as they completed their plans for the searches.

Similarly, Catherine A. Rinaldi, the general counsel for the Metropolitan Transportation Authority, was “intimately involved” in plans for searches on the Long Island Rail Road and Metro-North Commuter Railroad, according to Tom Kelly, a spokesman for the authority.

Ronald Susswein, an assistant attorney general in New Jersey, said that the New Jersey Transit searches would not be conducted arbitrarily. “This is not a criminal enforcement procedure,” he said. “We’re not trying to catch anyone. We’re trying to deter terrorism.”

While the majority of riders interviewed since Thursday said they supported the searches, a few have expressed concerns.

“For a split second, I thought that because of my skin complexion, they picked me,” said James Hamilton, 38, whose attaché case was searched yesterday at Herald Square. “I’m not sure if I had brown hair and blue eyes, that that would have happened.” He has black hair, dark brown eyes and a dark complexion.

Charles Wilson, 35, a schoolteacher whose bag was searched late Friday at Fulton Street in Lower Manhattan, said, “This is not going to make things better between the police and people of color.” Mr. Wilson is black.

Captain Adams, whose organization, 100 Blacks in Law Enforcement Who Care, held the news conference yesterday, said he believed that discrimination was likely in practice, if not intent. “You can say ‘no profiling,’ but when you have a police department that has a history of profiling, it is going to practice what it knows,” he said.

A police spokesman, Paul J. Browne, said he strongly disagreed. “These inspections are being conducted in a constitutional manner and have been met with enthusiastic cooperation by the overwhelming majority of riders we’ve come into contact with,” he said yesterday.

Ann Farmer, Patrick McGeehan and Jess Wisloski contributed reporting for this article.