The original Drudge headline said "search on for dirty bomb." It's since changed and this story appears above the fold:

The Massachusetts Emergency Management Agency has partially activated its emergency response team this afternoon after the FBI triggered a massive manhunt yesterday for six people who may pose a terrorist threat to the city of Boston. Public safety officials are now holed up in the emergency bunker in Framingham until the threat has passed.... Developing.

The Massachusetts Emergency Management Agency has been placed on standby, and public safety officials are meeting at the bunker, according to a published report Wednesday.

There have been reports that the FBI office in Boston received a call from an FBI office in California warning officials about a suspicious person that may be in the area, according to the Boston Herald's Web site.

No specific threats have been confirmed, but FBI agents in Boston have been put on alert, and officials started to gather at MEMA at about 1:30 p.m.

NewsCenter 5 and TheBostonChannel.com will have more information as it becomes available.

This one is a bit more specific:

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 Chineses (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''

More here:

"There is no specific threat or target indicated."

More at Boston.com

Update: Fox has finally picked up the story:

Boston on Alert for Four Terror Suspects

The FBI notified area law enforcement Wednesday to be on the lookout for four possible terror suspects who may be headed to Boston. 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.

Update: This story says they are searching for six men, not four.