The FBI (search) notified Boston area law enforcement Wednesday to be on the lookout for four Chinese nationals described as possible terror suspects who may be headed to the area.

Federal law enforcement officials in Boston said they had received a tip earlier in the day about an unspecified threat against Boston, and released photographs of the two men and two women they were seeking.

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

Authorities said none of the names had been on previous watch lists of terror suspects, and their whereabouts aren't immediately known.

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

Romney told reporters in Washington that he planned to be on a 9 p.m. flight back to Boston, and cautioned that the threats were unsubstantiated and uncorroborated.

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

Romney said that the state's threat level would not be raised, but more people would be on duty in the state's emergency management bunker in Framingham, about 20 miles west of 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."

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. No credible, specific terror threat has been identified in connection with the tip about suspects possibly entering the country from Mexico.

Brian Roehrkasse, spokesman for the Department of Homeland Security (search), said the agency was working with the FBI and "our partners in the intelligence community to analyze and assess recently received and uncorroborated information concerning a potential threat to Boston. This information has not been corroborated and was received from an unknown source of unknown credibility."

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.

FBI spokeswoman Gail Marcinkiewicz said the terror alert had not been raised for Boston.

"Basically, what you have here is information that we often get," she said. "It's uncorroborated at this time."

"It's been passed on to our law enforcement partners and we're working it aggressively," she said.

Peter Judge, a spokesman for the Massachusetts Emergency Management Agency, said representatives from six or seven state agencies were on duty at the state emergency bunker in Framingham — far fewer than the 30 or 40 that would be present during a full-blown emergency activation.

"We're just really at this point in a monitoring mode," Judge said. "We'll stay active through the night tonight, until this thing resolves itself."