6 sought after tip alleging 'dirty bomb'

Page 2 of 2 -- "None of the individuals that we're looking for is believed to be in Massachusetts," he said.

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Andrew Card, Bush's chief of staff, told reporters last night that he considered the threat to be based on "sketchy intelligence."

Federal authorities distributed fliers with pictures of the four Chinese nationals -- whom the tipster described as chemists, officials said -- to state and local law enforcement officers, as well as to airline representatives at Logan Airport. The pictures are believed to have come from the tipster in Mexico, who left them in a designated place for police to pick up.

The Boston Public Health Commission defines a dirty bomb as a conventional explosive laced with radioactive components, designed to contaminate a large area. It is not a nuclear bomb and does not involve a nuclear explosion, according to a description on the commission's website.

The tipster also told investigators at one point that the six were seeking nuclear material, according to two people involved in the investigation.

The Massachusetts Bay Transportation Authority tightened security across the transit system, though the T isn't expected to conduct random baggage searches as it did during the Democratic National Convention last summer.

"We are enhancing patrols through the core system and working in cooperation with the other law enforcement agencies," said John Martino, the deputy chief of the MBTA police.

Mayor Thomas M. Menino said at a brief news conference that he has ordered police to "look at certain sections of the city," but he urged calm and said people should continue their normal routines.

"If we have any information the public needs to know, we'll give it to them immediately," he said.

Boston police activated a central command post at headquarters that was used during the Democratic National Convention and added a half-dozen officers on overtime in downtown last night, according to a high-ranking officer briefed about the plan. Police officials also posted additional officers at the FleetCenter during the Celtics game, the officer said.

At a police briefing yesterday at District A-1 station near Government Center, a police intelligence official told officers that detectives in the department would be trained in the use of radiation-sensing devices, and he gave officers instructions about responding to a dirty bomb attack, along with descriptions of the suspects.

"They have modeling software they can use if it happens," said the officer, who attended the meeting along with representatives of the Fire Department and emergency medical services. "Within 3 minutes, they can tell us which direction the wind is blowing in, which streets to shut down. They said it is a threat and we're taking it seriously."

Specialists say the most likely dirty bomb scenario would involve an explosion that would kill only those people nearby.

"The good news is that the most accessible radioactive material and the most bombs that you would imagine somebody using to disperse will have only modest impacts on human life," said Dr. Graham Allison, director of the Belfer Center for Science and International Affairs at Harvard University and an assistant secretary of defense under President Clinton.

In recent months, federal authorities have grown increasingly concerned that Al Qaeda may be using the porous US-Mexican border to sneak terrorists into the United States.

Last April, a suspected Al Qaeda agent arrested in New York told officials of a plan to smuggle operatives from Mexico into the United States. Two months later, two alert border agents at a Texas airport detained a woman, Farida Goolam Mohamed Ahmed, because she was carrying a doctored South African passport. She was initially thought to be on a terrorism watch list barring entry into the country, but officials later said that wasn't the case.

Then in August, a senior Al Qaeda operative, Sharif al-Masri, was captured along the Afghanistan-Pakistan border and told interrogators of "Al Qaeda's interest in moving nuclear materials from Europe to either the US or Mexico," according to a bulletin that Department of Homeland Security officials provided to law enforcement agencies last November.

As thousands of people crowded into the FleetCenter last night for the Celtics game, a line of security guards patted down many of those entering and peeked into purses.

"Are we doing things differently today than we did last week? Absolutely," said John Wentzell, senior vice president and general manager of the FleetCenter. "There are things you can see; things you can't see. We're aware of the climate."

Shelley Murphy, Raphael Lewis, Andrea Estes, Bryan Bender, Steve Kurkjian, and David Abel of the Globe staff, and correspondent Scott Goldstein contributed to this report. Donovan Slack can be reached at dslack@globe.com.