The FBI has triggered a massive manhunt for six people -- four Chinese and two Iraqis -- who may pose a terrorist threat to the city of Boston, law enforcement officials briefed on the threat said today.

The six are suspected of having come into the United States from Mexico, and may have headed to New York and then to Boston, the target of a planned attack that could involve a lethal substance, possibly chemical or biological or explosive, three law enforcement officials briefed on the threat said.

The tip about the threat was given to the FBI by only one person, the officials said, and it had not been corroborated as of this afternoon...

Federal and state authorities are investigating a nuclear terrorist threat against Boston after a man calling from Mexico told California police that he smuggled two Iraqis and four Chinese over the border, the Boston Herald has learned.

``They got a call from across the border in Mexico to the California Highway Patrol and he said he brought two Iraqis and four Chinese (individuals) across the border and according to him, they stated soon to follow behind them would be some sort of material,'' said a law enforcement source familiar with the investigation.

``He refers to some sort of nuclear material that will follow them through New York up into Boston.''

According to the source, the caller has not identified himself and did not show up for a meeting with federal investigators in California but he did leave pictures of four Chinese men and some names at a ``drop'' site at the Mexico-California border.

``They were dropped by the source at a location. He literally threw them over a fence from Mexico to the U.S. side,'' said the source. ``There are pictures of the four Chinese and some names but just how accurate they are remains a question'' ...

...A company that trains explosive-sniffing dogs said it was alerted that the canines would be searching for a ``dirty bomb,'' a New York City law enforcement official said yesterday.

The Massachusetts investigator said much of the man's information sounds far-fetched and investigators have some doubts about the caller's validity because he has not identified himself...

...The source said there is speculation the caller may have been ripped off by illegal immigrants he helped over the border and is now trying to exact revenge.

``It's very weird. Even if (the Iraqis and Chinese) were going to do something why would they be blabbing to the yahoo smuggling them across the border? You have to wonder if they screwed him on a deal but you have to treat it seriously and the issue is how do you put it out to the public and not get everybody (in a panic)?'' ...