No arrests or investigations have resulted from security checkpoints MBTA police have run at T stations since Oct. 10, 2006, according to officials at the department.

While Deputy Chief Joseph O’Connor wouldn’t say how frequently the checkpoints were set up, “it’s safe to say they’re done a minimum of once a day … on a daily basis,” meaning there have been more than 1,420 checkpoints without an arrest or investigation.

Despite that, Massachusetts Bay Transit Authority Police Chief Paul MacMillan believes the checkpoints are valuable as part of a “layered approach” that also includes camera surveillance and plainclothes officers riding the system.

“Intelligence suggests terrorists do preplanning before they do an attack. And now, if on the day of their plan there’s an inspection, we have disrupted the plan and they have to regroup and do more planning,” MacMillan said.

There has never been a specific threat to Boston’s transit system or to a specific station, his deputy said, but mass transit is considered vulnerable to terrorism. There are stark examples of bomb attacks in Madrid in 2004, killing nearly 200, London in 2005, killing 52, and as recently as March, when a Moscow subway bombing killed 35. The Japanese cult Aum Shinrikyo killed 12 and sickened almost 6,000 people with Sarin nerve gas in a 1995 incident that was apparently done to bring on the apocalypse.

“We do know the threat to our system is ongoing,” O’Connor said. “We do know mass transit continues to be a target terrorist groups continue to attempt to exploit.”

In written testimony given in 2005 to a state Joint Committee on Public Safety and Homeland Security, Harvard security and terrorism expert Daniel B. Prieto cited the Congressional Research Service in stating that:

Fully one-third of terrorist attacks worldwide target transportation systems, and public transit is the most frequent transportation target. Analysis of more than 22,000 terrorist incidents from 1968 through 2004 indicate that attacks on land-based transportation targets, including mass transit, have the highest casualty rates of any type of terrorist attack.

Terrorists like mass transit because they are “open,” Prieto said.

The MBTA’s security checkpoints are set up to make the system less so.

Designed to be random

There are 278 Transit Police in the Boston system, 210 of who are in O’Connor’s Patrol Operations Division and handle the checkpoints, with a handful of officers needed at each one.

The checkpoints are set up at any entry point in the transit system, without pattern so “we could be [at the same station] tomorrow and could not be back for two weeks,” O’Connor said. “We could be there briefly or for a number of hours. We could be there a whole day. The idea is to be unpredictable.”

Transit officers stop commuters using a computer-generated formula that changes frequently and is similarly intended to follow an “unpredictable pattern, so if someone is watching, doing surveillance, they couldn’t stand at the station and walk through and be sure they weren’t the one to be picked,” he said.

A gloved officer uses a chemical swab on a commuters’ bag, and a machine is used to see if the swab picked up traces of a chemical. If there are traces, the commuter will be asked questions to determine what the machine could be detecting, O’Connor said. Only if the questions don’t prompt a reasonable response will people be asked to open their bags for a visual inspection, the officers said, which is why not even arrests for weapons or drugs have resulted from the inspections.

The process is estimated by O’Connor to take “under a minute. Generally, under 40 seconds.”

If someone doesn’t consent to a swab or search, they can leave the station but not continue into the system there to take a train or other mode of transportation.

Alarm fades, skepticism remains

MacMillan acknowledged that there is public skepticism about the value of the checkpoint technique, although it is endorsed by the Department of Homeland Security and Transportation Security Administration, and answered a specific way in which terrorists could get around it: “You could go to another station, true, but [a checkpoint] adds a deterrent because you don’t know where we’ll be.”

Terrorists “may get disrupted in their planning stages, and maybe something else comes to light” as they take longer to plan an attack, MacMillan said.

Cambridge Day made a Freedom of Information request July 28 asking about arrests, investigations and complaints arising from inspections. Spokesman Joe Pesaturo replied Thursday with the information, including that “seven informal complaints have been filed and responded to by e-mail.”

There was alarm about the system in the early days that resulted in ads from the American Civil Liberties Union of Massachusetts reminding T riders of their rights. During the 2004 Democratic National Convention in Boston, when the searches were first done, the American-Arab Anti-Discrimination Committee and the National Lawyers Guild sued but failed to stop them. Two years later, governor and presidential candidate Mitt Romney revived the searches.

While the sense of alarm has faded, the civil liberties group remains opposed to the searches, calling them counterproductive.

“We think of those searches as ‘pretend security’ or ‘security theater.’ While encouraging ordinary people to casually submit to being searched, they are unlikely to identify or stop determined attackers,” said Christopher Ott, communications director of the ACLU of Massachusetts.