A terrorist attack on chemical plants lacking enough federal oversight could cause mass casualties, a former homeland security adviser to the White House told senators on Wednesday.

"The chemicals that we are talking about today are in many cases identical to those used in the battlefield of World War I. They are enormously dangerous. They are produced in truly massive quantities, shipped and stored, in many cases, next to very dense urban populations, and (they) present, in my opinion, the single greatest danger of a potential terrorist attack in our country today," Richard Falkenrath (search), who left the White House in May, told the Senate Homeland Security and Governmental Affairs Committee.

New Jersey Democratic Sen. Jon Corzine (search) also testified before the panel, telling colleagues that in his state, 11 chemical plants are considered high-risk sites. One of them is just a mile and a half from the Holland Tunnel into New York City. He said hundreds of thousands of lives are vulnerable if something were to happen there.

Also attending the hearing was the head of the Chemical Safety and Hazard Investigation Board (search), the commission that investigates and determines the cause of deadly chemical accidents. Carolyn Merritt, who was appointed by President Bush, said that her board concluded that many communities may be vulnerable and inadequately prepared for a major chemical release caused by either an accident or an attack.

Merritt explained that protections today are not adequate enough to prevent a tragedy on the scale of one of the deadliest accidents in modern history — the Union Carbide Corporation (search) poison gas leak in Bhopal, India, in 1984 that left 3,000 dead and 200,000 permanently injured.

"The death toll of the Bhopal accident was extraordinary, but the accident itself was not," she said. "The amount of toxic material released — 43 tons — would fit comfortably into just one rail car."

The federal government estimates that more than 15,000 chemical facilities are located nationwide, including more than 100 in heavily populated areas. About 160 are considered particularly worrisome because they are so vulnerable to a terrorist attack. Such plants can store enough deadly chemicals to kill or injure hundreds of thousands of people.

John Stephenson, director of natural resources and environment at the Government Accountability Office (search), offered more alarming statistics.

"Chemical facilities could each potentially put at risk more than 1 million people to a cloud of toxic gas. About 600 (plants) could each potentially threaten from 100,000 to one million and about 2,300 such facilities could each potentially threaten from 10,000 to 100,000," Stephenson said.

Wednesday's hearing revealed that a growing number of government officials believe the current policy to protect chemical plants — one that relies on the owners and not the government to set the security standard — has to be reconsidered.

Department of Homeland Security officials told FOX News on Tuesday that DHS has no legal authority to force plants to ramp up security. But department spokeswoman Michelle Petrovich said DHS is not waiting for legislation. Inspectors have reviewed security at more than 160 of the 300 plants "of immediate concern" and DHS is helping the most vulnerable shore up security through federal grant programs.

But industry officials who oppose federal regulations like those now placed on commercial airports and nuclear facilities, said voluntary security measures are the best approach.

"Our members are committed to securing their facilities against acts of terrorism," Rob Carver of the Synthetic Organic Chemical Manufacturers Association (search), which represents 300 chemical companies, told The Associated Press.

Stephen Flynn, a counterterrorism expert at the Council on Foreign Relations (search), countered that terrorists are looking for the path of least resistance.

"Al Qaeda or one of its many radical jihadist imitators will attempt to carry out a major terrorist attack on the United States within the next five years," Flynn said.

Merritt said that it was only a matter of luck — prevailing winds and rainstorms — that recent chemical accidents in the United States did not cause more mass casualties.

A car that derailed in Graniteville, S.C., in January killed 10 people after several tons of chlorine were released. If a similar incident had happened in a densely populated area, experts say thousands could have been poisoned and killed.

The nation's capital just won support from a federal court that upheld a new ordinance banning rail shipments of toxic chemicals through Washington, D.C., where a terrorist attack on such a train could kill 100,000 people.

Falkenrath, now a visiting fellow for foreign policy studies at The Brookings Institution, said he is disappointed with the laxity of Bush administration policy on chemical plant security.

But, he added: "Regretfully, some portion of this responsibility clearly belongs to me."

FOX News' Catherine Herridge contributed to this report.