Leonard A. Cole, a dentist who became an expert on biological weapons and who chronicled in troubling detail a secret U.S. Army program that turned millions of Americans into unwitting germ-warfare guinea pigs in the 1950s and ’60s, died on Sept. 18 in Ridgewood, NJ. He was 89.

Dr. Cole’s dental practice was firmly established when he began a second career s a political scientist, looking into the clandestine military tests. The program, which ran from 1949 until President Richard M. Nixon halted it in 1969, involved releasing ostensibly harmless bacterial and chemical agents in the New York City subway, over the skies of San Francisco, and in dozens of other places to test the country’s vulnerability to biological and chemical attacks. His book, Clouds of Secrecy: The Army’s Germ Warfare Tests Over Populated Areas,” published in 1988, offers an in-depth examination of the Army program.

Dr. Cole retired from dentistry in 2000 but continued to write books. To balance his various endeavors, Dr. Cole said in an interview with the online publication Authority Magazine last year that he gave his “undivided attention” to whatever he was doing at the moment. He said that a friend had once told him he was the best dentist among political scientists, and the best political scientist among dentists.