Report: Boston Dirty Bomb Threat

An anonymous tip Wednesday is prompting an East Coast manhunt for four Chinese scientists and two Iraqis said to be planning to detonate a dirty bomb, public safety officials briefed on the threat told the Boston Globe.

A specific but unreliable tip is keeping law enforcement officials on careful lookout specifically for the four Chinese nationals whose names and images are known and who may be in or headed to the New York or Boston areas.

Massachusetts Governor Mitt Romney returned home from the inaugural festivities in Washington, D.C., due to the terror threat, which he said comes from a "very questionable source." Romney told a news conference in Boston that he's been briefed periodically on the situation, and he can see no cause for alarm.

Law enforcement officials in California received a call from across the Mexico border warning of possible attacks. The caller warned of six potential terrorists sneaking into the United States and that "some sort of nuclear material that will follow them through New York up into Boston."

Federal prosecutor Michael Sullivan says more has been learned about the Chinese nationals in the photos released last night. But none of the new information has raised any new alarms. Officials say they would simply like to locate and speak with the individuals.

The terror alert has not been raised and both Romney and Boston Mayor Tom Menino have urged residents to go on with their plans.

New York Mayor Michael Bloomberg mentioned the possible terror alert in a late Wednesday afternoon news conference at City Hall. He didn't appear worried about the situation.

Homeland Security officials are working with the FBI to confirm whether the four suspects entered the United States from Mexico.

The U.S. Attorney's office released photographs of the two men and two women being sought to law enforcement, the Massachusettes Joint Terrorism Task Force and the media. Authorities said none of the names had been on previous watch lists of terror suspects.

The investigation stirred a frenzy of media reports and prompted Gov. Mitt Romney, who had gone to Washington to attend Thursday's presidential inauguration, to decide to return to Massachusetts later Wednesday.

"We have had threats in the past. We take them seriously, even when they're not corroborated," he said.

Although the FBI and local law enforcement aren't talking details, the Boston Globe reported that the source of the tips wouldn't meet with investigators, but did agree to drop off vital information, including the photographs of the four Chinese men and women, at a location along the U.S.-Mexico border.

Romney said the state's threat level would not be raised, but more people would be on duty in the state's emergency management bunker outside Boston.

"To assure the people of Boston and Massachusetts that it is safe to be at home, I am going to be sleeping in my bed in Massachusetts tonight and I feel perfectly safe doing so," Romney said. "In the very remote circumstance that my attention is needed, I will be able to respond on an immediate basis."

New Hampshire Governor John Lynch also left Washington, to be on standby to help just in case, reports CBS News Correspondent Kelly Cobiella.

A federal law enforcement official, speaking on condition of anonymity because the matter remains under investigation, told The Associated Press that the FBI's joint terrorism task force has not yet corroborated a tip that the suspects entered the United States through Mexico in recent days, possibly bound for Boston.

The official stressed the tip is one of many from around the country that routinely are forwarded to local task forces for further investigation.

Brian Roehrkasse, spokesman for the Department of Homeland Security, said the agency was working with the FBI and intelligence officials to investigate the situation. Roehrkasse said he was not aware of any stepped-up federal security measures in Boston, such as extra patrols or measures taken at Logan International.

"Basically, what you have here is information that we often get," FBI spokeswoman Gail Marcinkiewicz said.

In a joint statement issued by Massachusetts U.S. Attorney Michael Sullivan and Kenneth Kaiser, an agent in charge of the Boston FBI office, the suspects were identified as: Zengrong Lin, Wen Quin Zheng, Xiujin Chen, Guozhi Lin.

"All law enforcement agencies are working to together to address the issue," said the statement.