A sketchy threat to blow up vehicles full of explosives prompted authorities Tuesday to close one of the busy tunnels underneath Baltimore's harbor and partially shut down the other. One person who may have been connected to the threat was arrested on immigration charges, a law enforcement official said.

Traffic was allowed to resume by early afternoon after being diverted for nearly two hours, but the FBI continued to investigate.

The four-lane Baltimore Harbor Tunnel was closed around midday, and the eight-lane Fort McHenry Tunnel (search) was reduced to one lane in each direction. The tunnels, both about 1.4 miles long, carry traffic between Washington and the Philadelphia and New York City areas.

A federal law enforcement official, speaking on condition of anonymity, said a man in custody in the Netherlands was the source of information about the threat.

Authorities questioned the credibility of the threat but looked for several men who the source said would drive explosives-laden vehicles into the tunnel, said another federal law enforcement official, who also spoke on condition of anonymity.

"While the information was somewhat specific, to date the intelligence community has not found evidence that corroborates the information," FBI agent Richard Kolko (search) said.

Shortly before the tunnels were reopened, authorities made about a half-dozen raids in the Baltimore area, said city Police Commissioner Leonard Hamm.

Four people were arrested on immigration charges at businesses with Middle Eastern connections, said a federal law enforcement official, who spoke on condition of anonymity.

One of them was arrested as a result of information supplied by the FBI's Joint Terrorism Task Force (search), which was heading the tunnel investigation, the source said. That arrest, made at a pizza shop, may be connected to the threat, the source said.

State and local authorities closed the tunnels "out of an abundance of caution," said Jim Pettit, a spokesman for Gov. Robert Ehrlich's homeland security office.

On Oct. 6, a threat to blow up New York's subway system prompted authorities to tighten security underground. Several days later, officials said there was no clear evidence to back up the threat, and security was scaled back.

An average of 70,000 vehicles a day went through the Baltimore Harbor Tunnel (search) in 2004. The Fort McHenry Tunnel averaged 116,000 vehicles a day.