The Border Patrol quietly has stopped its routine practice of searching for illegal immigrants on buses, trains and airports near the northern border, preventing agents from using a longtime, effective enforcement tool.

The order has not been publicized, but two Border Patrol agents described it on condition of anonymity. The union that represents Border Patrol agents planned to issue a news release about the change Monday. Other agents said field offices began receiving the order last month — soon after the Obama administration announced it would allow many illegal immigrants to remain in the country while it focuses on deportations of criminals.

The routine bus, train and airport checks typically involved agents milling about and questioning people who appeared suspicious, and long had been criticized by immigrant-rights groups. Critics said the tactic amounted to racial profiling and violated travelers' civil liberties.

But agents said it was an effective way to catch unlawful immigrants, including smugglers and possible terrorists who had evaded detection at the border, as well as people who had overstayed their visas. Those who evade detection often head quickly for the nearest mass public transportation in hopes of reaching other parts of the country.

The number of Border Patrol arrests nationwide has been falling — from nearly 1.2 million in 2005 to 463,000 in 2010, and 97 percent of those were along the southern border, according to the Department of Homeland Security's Office of Immigration Statistics. The Border Patrol made 7,431 arrests near the northern border last fiscal year, but it was not immediately clear how many stemmed from routine transportation checks. Of 673 arrests in the Blaine sector, roughly 200 were from such checks, according to a Washington-state-based agent who has been with the Border Patrol for more than 20 years.

Gene Davis, a retired deputy chief in the Blaine sector, noted that a check of the Bellingham bus station in 1997 yielded an arrest of Palestinian Gazi Ibrahim Abu Mezer. He skipped out on a $5,000 bond — only to turn up in Brooklyn, where New York police shot him as he prepared to bomb the city's subway system. Davis also noted that would-be millennium bomb suspect Ahmed Ressam was arrested in Port Angeles in late 1999 when he drove off a ferry from British Columbia in a rented car full of explosives.

"We've had two terrorists who have come through the northern border here — to put these restraints on agents being able to talk to people is just ridiculous," Davis said. "Abu Mezer got out, but that just shows you the potential that's there with the transportation checks."

Halting the practice has baffled other former and current agents as well, especially those in stations along the northern border — from Bellingham to Houlton, Maine — where the so-called "transportation checks" have been the bulk of their everyday duties. The Border Patrol is authorized to check vehicles within 100 miles of the border.

Doug Honig, spokesman for the American Civil Liberties Union of Washington, welcomed the news.

"If the Border Patrol is indeed not boarding buses and trains and engaging in the random questioning of people, that's a step in the right direction," he said. "People shouldn't be questioned by government officials when there's no reason to believe they've done anything wrong."

Those who have received the orders said agents still may go to train and bus stations and airports if they have specific "actionable intelligence" that an illegal immigrant has entered the country. Even in such cases, an agent in Washington state said the new directive requires clearance from Washington, D.C., headquarters.

A Customs and Border Protection spokesman, Bill Brooks, repeatedly insisted any shift in enforcement tactics does not amount to a change in policy. He said local commanders retain authority to pursue illegal immigrants near the border and at transportation hubs.