Despite questions about the credibility of a threat to detonate vehicles full of explosives, officials said they had little choice but to close one of the busy tunnels underneath Baltimore's harbor and partially shut down the other for nearly two hours.

One person who may have been connected to the threat was arrested on immigration charges, a law enforcement official said.

The FBI had found no evidence to corroborate the threat by Tuesday evening, said agent Kevin Perkins of the FBI's Baltimore field office.

"It's unfortunate, the day and age in which we live, that we have to follow every single one of these threats," Perkins said.

The four-lane Baltimore Harbor Tunnel (search) 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.

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

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 said.

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

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.

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

On Oct. 6, a threat to blow up New York's subway system prompted authorities to tighten security underground. The city's announcement of that alleged plot led to jostling between city officials and homeland security officials in Washington, who downplayed the threat.

Several days later, officials said there was no clear evidence to back up the threat, and security was scaled back.

California Rep. Jane Harman (search), the senior Democrat the House intelligence committee, said events in Baltimore and New York point to problems in the nation's revamped intelligence network.

"In both cases, you have local responders acting on what they believe to be credible and specific information. I think they made the right call. Then you have inside-the-Beltway operatives in our federal intelligence agencies second-guessing them and in the New York case, undercutting them. This is unhealthy and very confusing to the public," she said.