BANGOR, Maine — A U.S. Border Patrol checkpoint on Interstate 95 that began Tuesday and resumed Wednesday was part of the federal agency’s overall mission to stop terrorists and their weapons from entering the country.

In this case, however, the particular focus was on ferreting out any illegal aliens who might have entered the United States unlawfully through its northern border, Deputy Chief Patrol Agent Alfredo Casillas of the U.S. Customs and Border Protection’s Houlton Sector said Wednesday afternoon.

“This is just one of what I call the tools in our toolbox,” Casillas said of the checkpoints conducted this week.

Though Casillas declined to say if the checkpoint would be operating today, he did say that agents had stopped about 1,000 vehicles a day earlier this week to determine if the drivers and passengers were U.S. citizens and if not, if they were here legally.

Casillas said that I-95 is an ideal location for such an operation because it is a “choke point” for people — including illegal aliens — who have entered the United States through its northern border and are heading south.

While checkpoints are under way, agents also keep an eye out for other illegal activities, including stolen vehicles, smuggling and contraband, including drugs, he said.

The checkpoint operation involved Border Patrol agents and trainees, dogs that can sniff out hidden people and drugs, and an aircraft that flew over the area looking for any vehicles driven by motorists who might have found out about the checkpoint and taken another route in an effort to bypass it.

That’s because illegal aliens, smugglers and those involved in the illegal drug trade have been known to send out “scout vehicles” that through radios or cell phones can alert their accomplices ahead of time, thereby enabling them to avoid detection, Casillas said.

The checkpoint near Sherman was among many that Border Patrol agents conduct from time to time, typically without notice, he said.

“It does catch those who are going down the highway off guard but if we [announced plans ahead of time] the checkpoints would be much less effective,” he said.

He also couldn’t divulge if the agents encountered any aliens or contraband this week, but he did say checkpoints over the last year or two had turned up “several pounds” of OxyContin that otherwise would have wound up on the streets.

The timing of a checkpoint is based on several factors, including intelligence the agency receives from its own personnel, from other state and federal agencies in the United States and Canada and from the public on both sides of the border, Casillas said. He also said that the timing is driven by available manpower, federal and state safety regulations and weather considerations.

Casillas recommends that motorists who encounter a Border Patrol checkpoint simply use their common sense.

“I know some folks are inconvenienced but usually it takes a mere one or two minutes and you’re on your way,” he said.

Occasionally, however, vehicles carrying several passengers or motorists who warrant a closer look — such as those who appear unusually nervous or who are driving rental cars — are pulled over so as not to impede the flow of traffic.

“Border Patrol, we’re all part of the community and one of our goals is to make our communities safer. Cooperating and just letting us do our job makes it that much easier for the traveling public,” Casillas said.