Homeland Security Secretary Tom Ridge raised the threat level to "high" in portions of Washington and New York on Sunday after reports that the bin Ladin network planned to use truck or car bombs against financial targets, including New York-based Citigroup and Prudential Financial buildings in northern New Jersey.
Speaking on US television early on Monday Ridge said, without giving a specific time or date for the possible attacks, he would rate the quality of the information, which came from multiple, highly-reliable sources, a "10."
"Across the board, everyone that has reviewed the sources has concluded that it is as reliable a group of sources as we have ever seen before," Ridge told NBC's Today Show programme.
Ridge claimed the threat was
sophisticated and complex
As a precaution, authorities closed New York's Holland Tunnel to commercial traffic heading into the city beginning on Monday. The tunnel links an area of Manhattan (where several of the financial institutions are located) to New Jersey.
In Newark, New Jersey, police set up metal fences around the headquarters of Prudential Financial, blocked off two streets and armed themselves with assault rifles.
Ridge announced on Sunday the threat level was being raised to Code Orange, or "high", from Code Yellow, or "elevated" in the financial services districts of New York, Washington and the northern part of New Jersey. The rest of the United States remains at an "elevated" threat level.
Police will increase checks on large vehicles entering downtown Washington, and bomb-sniffing dogs will check the area around the buildings, Police Chief Charles Ramsey said.
Washington police also increased security at the Bureau of Engraving and Printing, one of the sites where US currency is printed, as well as the Federal Reserve headquarters.
An intelligence official told reporters the information being gathered included details on escape routes, security cameras, the best place to park vehicles with explosives, pedestrian and traffic patterns, and even the degree of garage ramp inclines.
"Across the board, everyone that has reviewed the sources has concluded that it is as reliable a group of sources as we have ever seen before"
secretary, homeland security, USA
Police warned building managers and corporate security personnel to watch for vehicles that could be rigged with explosives and for chemical agents placed in ventilation systems.
Ridge told NBC television on Monday the elaborate plot demonstrated "the sophistication, degree of complexity and resolve that our enemies have."
The alert increase resulted from a series of events that began with the capture of an al-Qaida computer engineer in Pakistan in mid-July, the New York Times reported on Monday.
Pakistani officials arrested Muhammad Naaim Nur Khan, a 25-year-old computer engineer, on 13 July, according to the Times. The suspect is believed to have used and managed an al-Qaida communications system where information was transferred via coded messages, according to the Times.