Last year, Dr. Tucker stepped down after nearly 15 years as a research fellow in Washington at what is now the Monterey Institute’s James Martin Center for Nonproliferation Studies. At his death, he was awaiting a security clearance for a new job at the Department of Homeland Security.

A former editor at the journal Scientific American, he wrote authoritative histories on chemical warfare and the eradication of smallpox. In the early 1990s, he worked on arms control and nonproliferation matters at the State Department and the congressional Office of Technology Assessment.

By many accounts, Dr. Tucker possessed a scientist’s probing mind and a policy wonk’s fluency on national security issues. These traits earned him a trusted reputation on Capitol Hill and made him a key source to journalists seeking a credible opinion on biological and chemical weapons.

“Jonathan was a rare breed in that he knew the science of the issue, which was really complicated, and also knew the policy side,” said Paul Carroll, program director at the Ploughshares Fund, a nonproliferation group. “He was one of really a handful of people that could talk to both of these audiences, to both chemists and diplomats.”

In 1995, Dr. Tucker served as a United Nations weapons inspector in Iraq and helped comb laboratories there for lethal germs, noxious gases and other toxic substances. He used his firsthand knowledge of Iraqi leader Saddam Hussein’s chemical weapons program to advise the U.S. government before the invasion of Iraq in early 2003.

Dr. Tucker provided expert testimony to Congress on how he thought Hussein could potentially use his alleged arsenal against the American assault. He said U.N. representatives routinely discovered and destroyed hidden caches of Iraqi biological and chemical weapons after the 1991 Persian Gulf War.

Based on his research, Dr. Tucker told The Washington Post in 2003 that Hussein “may very well use whatever he has” in a “last-ditch defense situation.”

Dr. Tucker added that the artillery systems Hussein used in the Iran-Iraq war of the 1980s had since become obsolete and would not effectively distribute germ weapons.

Hussein “could contaminate areas,” Dr. Tucker said, but any possible usage of the weapons would “only slow the oncoming forces down” because U.S. troops had protective suits.

Ultimately, the precautions were unnecessary. Saddam had dismantled his weapons program years earlier.

Jonathan Brin Tucker was born in Boston on Aug. 2, 1954. He was a 1972 graduate of the private Phillips Academy in Andover, Mass.

He graduated from Yale University three years later with a degree in biology. He received a master’s degree from the University of Pennsylvania before earning a doctorate in nonproliferation studies from the Massachusetts Institute of Technology.

(Jonathan Winer) - Jonathan B. Tucker, 56, an expert on biological and chemical weapons and an influential nonproliferation advocate, was known as much for his scholarship as for being a policy wonk.

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