BALTIMORE -- Authorities have arrested a man they say may be connected to the threat that prompted the temporary shutdown of the Harbor Tunnel and restrictions on the Fort McHenry Tunnel in Baltimore.

The suspect was one of four people arrested on immigration charges after several raids were conducted at businesses with Middle Eastern connections.

A federal law enforcement official said the source of information regarding a threat that shut down one of the busy tunnels underneath Baltimore's harbor was a man in custody in the Netherlands.

A second FBI agent said the information in the threat was "somewhat specific," but he said the information was not corroborated.

The four-lane Harbor Tunnel, which has been reopened to traffic, was closed for two hours around midday. Traffic in the nearby eight-lane Fort McHenry Tunnel was reduced to one lane in each direction.

The tunnels carry traffic between Washington and the Philadelphia and New York City areas.

Maryland Transportation Authority Chief Gary McLhinney said the order to reopen the tunnels arrived at about 1:10 p.m.

Federal officials said that they may or may not be searching for a specific vehicle, reported Baltimore television station WBAL, but pictures showed police were stopping and examining larger vehicles around the Fort McHenry tunnel.

Police were acting on information concerning the Baltimore tunnels that was gathered over the past several days, WBAL reported. The station said police emphasized that they were showing "an abundance of caution" in undertaking the operation.

Images from WBAL's helicopter showed several officers walking around with weapons drawn at the mouth of the Fort McHenry tunnel. During the shutdown, traffic was a complete standstill leading to the Harbor Tunnel. Police were diverting traffic off Interstate 895.

The FBI also conducted an operation at a separate location in conjunction with this security search and inspection, the television station said, but no details were available.

Henry Fawell, a spokesman for Maryland Gov. Robert Ehrlich, said there was a joint federal, state and local security operation under way and Ehrlich was closely monitoring the situation. He declined to comment further.