BALTIMORE, Maryland (CNN) -- Federal agents were questioning "a couple" of people Tuesday in connection with a terror threat that prompted Baltimore authorities to temporarily close one of two downtown tunnels under Baltimore Harbor and restrict traffic through the other, U.S. officials said.

The restrictions were put in place out of what a state police official called "an abundance of caution," although the FBI said it had not been able to corroborate the reported threat.

They were imposed as a result of a "threat of undetermined credibility to an unspecified tunnel in the Baltimore area," said Kevin Perkins, a spokesman for the FBI's Joint Terrorism Task Force.

The FBI and other law enforcement agencies are looking for at least six men they were told are part of the alleged plot, U.S. officials said. The original source of the intelligence said those involved were Egyptian, but officials did not confirm the nationalities of those being sought.

A federal government official said the source of the threat intelligence has provided some useful information in the past. But Perkins said agents were interviewing "certain individuals in an effort to determine the credibility of the information."

According to multiple U.S. officials, the Baltimore alert was triggered by a report that a shipment of explosives was heading into the city's harbor disguised as cocoa. The explosives then would have been used to build a truck bomb to be detonated inside the tunnel, the officials said.

The latest threat came nearly two weeks after a similar warning prompted a security alert on New York's subway system -- and the warning later was determined to have been a hoax, government sources said.

Federal, state and local authorities began investigating reports of a tunnel plot two weeks ago, and police had tactical squads ready to respond to an attack, Maryland Gov. Robert Ehrlich said.

He said the state's alert level was not raised because "you don't want to give advance notice to the bad guys."

"It was a very long night last night, I have to tell you," Ehrlich told CNN.

The Baltimore Harbor Tunnel on Interstate 895 was closed, while traffic on Interstate 95 through the Fort McHenry tunnel was limited to one lane in each direction.

The measures lasted less than two hours and were lifted shortly after 1 p.m.

Police with search dogs stopped trucks headed for the tunnels and inspected their undercarriages with mirrors.

Gary McLhinney, chief of the Maryland Transit Authority police force, would not disclose what officers were looking for.

But he said, "We have not found anything to cause us any great concern at this point at our tunnels."

U.S. intelligence officials said there were serious questions about the Baltimore tip from the outset, and those doubts have persisted. One official said the information came from a "single uncorroborated source overseas," but added he was not criticizing Maryland authorities for the closures.

McLhinney made no apologies for the steps taken Tuesday.

"Our No. 1 priority is the safety of the citizens of Maryland and those who travel on our roadways, and we will always err on the side of public safety," he said.