Homeland Security Secretary Tom Ridge (search) raised the threat alert level Sunday for the financial sectors in Washington, D.C., New York and northern New Jersey.

At a press conference intended to update Americans about protective measures being put into place in response to threats from Al Qaeda (search), Ridge said three targets have been specifically threatened: The World Bank and IMF in Washington, D.C., the Prudential Building in Newark, N.J. and the New York Stock Exchange.

Officials said that in the last 24 to 36 hours, intelligence operatives have received very specific information showing that Al Qaeda has done very detailed surveillance on targets

It was not immediately known what intelligence was leading the government to consider such action, officials suggested that Al Qaeda may possibly use car or truck bombs. New York City Police did not comment on any intelligence, but they urged extra security precautions at various city buildings.

Law enforcement officials say that Al Qaeda planned to send terrorists across the Mexican border in to the United States to conduct suicide attacks in the city. The attacks could come between now and Election Day. Republicans are planning their national convention in New York between Aug. 30 and Sept. 2.

New York's status has remained at orange, indicating the second highest risk of terrorist attack, since Sept. 11, 2001, and a change to red would not mean dramatic changes in the city, said a Homeland Security official, who spoke on condition of anonymity. However, a threat on financial institutions could have the impact of depressing stock trades on Wall Street on Monday, allowing terrorists to impact the U.S. economy.

Washington and elsewhere are on yellow alert, or elevated status of risk; that is in the middle of the five-color scale.

A Winding Trail of Terror?

The latest warning appears to be linked to the July arrest in Texas of a Pakistani woman accused of illegally crossing into the United States. Law enforcement accounts indicate she had an altered South African passport along with $7,000 in cash and an airline ticket to New York.

U.S. law enforcement authorities say they believe crime syndicates within the South African government are selling illegal passports for as little as $77 apiece.

The woman, Farida Goolam Mohamed Ahmed, 48, traveled from Johannesburg through Dubai, United Arab Emirates, to London and on to Mexico City. Authorities said she later told them she was smuggled into the United States from Mexico by crossing the Rio Grande.

Authorities say Ahmed's name appeared on an FBI watch list and it surfaced during an investigation of an overseas terrorist incident. Homeland Security officials have warned that Al Qaeda could use women and non-Arabs as operatives to avoid detection.

Rep. Solomon Ortiz, the ranking Democrat on the House Armed Services Subcommittee on Readiness, said "very credible" people told him Ahmed has traveled within the United States more than 250 times.

A senior official at U.S. Border Patrol told Fox News that a culmination of several pieces of information led to the increase. That included the arrest of Ahmed, but she may not be the key to the raised alert, just a complement to data that had already been obtained.

Ahmed's court-appointed attorney said his client is not charged with any terrorist activity and does not have a criminal record. However, she was denied bond on Tuesday.

The U.S.-Mexico border (search) is an area of concern to congressional members as well as U.S. officials. Sen. Joe Biden, D-Del., said in the War on Terror, more efforts are needed to reduce vulnerabilities along the border.

"Some progress has been made under border security, but we haven't implemented" enough measures, Biden said. "We shold be focusing on a line that is clearly drawn here on Earth. They relate to borders, they relate to rails, they relate to 101 nuclear power plants that exist and we're doing woefully little on any of those things."

Biden, the ranking Democrat on the Senate Foreign Relations Committee, said Al Qaeda knows all the prime targets it wants to hit in the United States. Ridge has pointed out before that Al Qaeda has to be right or lucky once to carry out an attack, but U.S. officials have a much heavier burden. To prevent an attack, they must be right all the time.

New York City police officials have warned businesses in the city to take extra precautions at work, including checking employee identification cards and updating emergency plans.

It also gave some things to look out for, including unanticipated deliveries or maintenance work, people taking unusual video or photographs, and visitors claiming to be lost or looking disoriented. The warning also said bomb threats may be used to evaluate emergency response time.

Fox News' Kelly Wright and Anna Stolley and The Associated Press contributed to this report.