The United States has spent as much as $5 billion since 1991 to help secure the former Soviet Union's vast nuclear, chemical and biological arsenal, but U.S. officials say they still can't account for all the weapons.

Since the Sept. 11 terrorist attacks, the United States should be very concerned that some of these Soviet weapons of mass destruction may have slipped into the wrong hands, said Sen. Dick Lugar, R-Ind.

"That is the worst-case scenario," he said. "That is the one thing we must make certain did not happen."

Lugar co-authored legislation a decade ago that launched the U.S. effort to safeguard the Soviet arsenal during the political, economic and social chaos that surrounded the dissolution of the Soviet Union. Since then, he said, roughly half the Soviet nuclear warheads have been destroyed.

The secure disposal of the materials that those weapons use for nuclear explosions -- plutonium and highly enriched uranium -- is still difficult, he said, and the progress of securing the chemical and biological stockpiles has proceeded far more slowly than the destruction program.

The United States has upgraded security systems that cover about one-third of the almost 700 tons of weapons-grade nuclear material identified as at risk of theft or diversion from Russia, according to the General Accounting Office, the investigative arm of Congress.

Restrictions imposed by Russia have kept the U.S. Department of Energy from installing security systems at about 100 buildings that contain hundreds of metric tons of nuclear material, according to a February GAO report. The report cites a wide-open gate at one Russian nuclear facility.

Ken Alibek, a former top scientist in the Soviet biological weapons program who came to the United States in 1992, said economically struggling Soviet weapons scientists pose the greatest threat.

Finding raw materials for biological weapons is easy because each country has its own pathogenic microorganisms, Alibek said, but such materials are worthless without the ability to transform them into weapons.

"In the field of biological weapons, the real threat is knowledge," he said.

The State and Defense departments have programs to put Soviet weapons scientists to work on beneficial research to reduce the risk they will be recruited by terrorists or smaller nations out to develop mass-destruction armaments.

Alibek said the money from those programs doesn't always go to the right people in the biological weapons area. Hundreds of bioweapons scientists have received not a penny.

In addition, he said, security remains lax at some Soviet facilities that work with deadly biological agents.

Chris Kessler, spokesman for the State Department's nonproliferation bureau, said the agency "has no reason to believe that Russia or any Central Asian country has been the source of anthrax or any other pathogen" used in the mail attacks in the United States. He declined to elaborate, citing the ongoing investigation.

Former Energy Secretary Bill Richardson said U.S. programs to support Soviet weapons scientists are a good start but are insufficient given the magnitude of the problem. An international effort is needed, he said.

"Now, hopefully, the warming of relationships between the U.S. and Russia will enhance cooperation," he said, "but you still cannot prevent a hungry Russian scientist who cares about feeding his family from defecting for the right price to Iraq, Iran, North Korea or even" Osama bin Laden.

Former Sen. Sam Nunn, D-Ga., who authored the Soviet nonproliferation legislation with Lugar, said Americans are safer than they were during the 40 to 50 years that the threat of a Cold War-driven nuclear holocaust hung over their heads.

Still, he said, the United States cannot be sure some weapons and expertise have not leaked out of the former Soviet Union.

"We'll never be sure, and we'll never be absolutely safe," said Nunn, who now heads the Nuclear Threat Initiative foundation.