National security agencies descended upon Boston Wednesday in response to an anonymous tip the FBI received suggesting that four Chinese nationals may be connected to a “dirty bomb” possibly located in the downtown area.

But Harvard officials said yesterday that the University does not appear to be at risk.

The FBI launched a massive manhunt in search of the four Chinese citizens, all of whom are supposedly chemists. It added the names of nine Chinese people and another man to list of suspects last night, according to the Associated Press.

The Department of Homeland Security, the Department of Public Health, the National Guard, and Massachusetts State authorities have shacked up in an emergency bunker in Framingham in order to plan for a possible threat—the same bunker in which they met after Sept. 11, 2001—according to the Boston Globe.

The Associated Press also reported that the FBI is searching for two Iraqi nationals also wanted in connection to the threat and that the original four suspects entered the United States from Mexico.

Regional and national security agencies have responded quickly to the threat, and Mass. Gov. W. Mitt Romney cut short his trip to Washington, D.C. where he planned to attend President Bush’s second inauguration.

The Boston Police Department also positioned extra officers, trained to pinpoint and respond to radiological detonation, in the downtown area yesterday.

According to Dillon Professor of Government Graham T. Allison, who served as assistant secretary of defense for policy and plans during the Clinton administration, a “dirty bomb” is a conventional explosive device used to disperse radioactive material.

But Allison said the threats have largely been exaggerated by unconfirmed reports.

“If this were a dirty bomb, it would be a bomb in a trash can or in a car, let’s say in Boston Common, and the blast would be identical to a car bomb. The impact on [Harvard] physically is nearly zero,” he said. “The radioactivity that would be dispersed by the bomb would most likely be a local event.”

Allison said the unusually dramatic response of authorities is largely due to the coincidence in timing between the threat and yesterday’s inauguration.

“We get information like this all the time. We take it seriously, and we go out and investigate it to make sure there’s not any truth to it,” said Gail Marcinkiewicz, a local FBI spokeswoman. “I think the important thing to remember is that this information is not corroborated.”

The threat, according to Allison, is not expected to seriously affect Harvard’s security.

“I would say the likelihood, from the evidence we have so far, that a bomb would go off in Cambridge is very, very slim. And since the story was about Boston rather than Cambridge, I would say that is an additional reason to assume it would not happen here,” he said.

The Harvard University Police Department (HUPD) could not comment on the details of the force’s response to the recent threat.

“We have been in contact with our partners in the state and local law enforcement agencies to see if there’s any specific threat against the University,” said HUPD spokesman Steve Catalano. “At this time, no specific threat against the University has been made.”

“Obviously, our officers are being vigilant and we encourage the community to call us if they observe any suspicious activity,” Catalano added.

—Staff writer Eduardo E. Santacana can be reached at esantac@fas.harvard.edu.