Harold Agnew, a leading figure of the nuclear age who helped design the first atomic bomb as a member of the Manhattan Project, led efforts after World War II to make the weapons more secure and championed the development of nuclear power, died Sept. 29 at his home in Solana Beach, Calif. He was 92.

His family said in a statement that he had chronic lymphocytic leukemia.

In a career that spanned more than 50 years, Dr. Agnew held a unique vantage point on the nuclear era. A physicist who trained under Enrico Fermi, he helped build the world’s first reactor, flew alongside the Enola Gay when it dropped its devastating load on Hiroshima and headed Los Alamos’s weaponry division during a prolific postwar period when the lab developed many new weapons, including the thermonuclear warhead for the Minuteman intercontinental ballistic missile.

Dr. Agnew was director of the Los Alamos National Laboratory in New Mexico from 1970 to 1979.

“Harold was an innovator,” said laboratory historian Alan Carr. “The vast majority of weapons in the nuclear stockpile were designed at Los Alamos, and Harold had a hand in designing most of them — I’d say about 75 percent.”

This 1970 photo provided by the Los Alamos National Laboratory shows Norris Bradbury, left, and Harold Agnew, right, the second and third directors of Los Alamos National Laboratory. (HOPD/AP)

In an interview with the BBC in 2005, Dr. Agnew said he remained an unapologetic hawk throughout his life, even after many of the leading scientists of the early nuclear era, including Albert Einstein and J. Robert Oppenheimer, expressed moral reservations about the bomb and its consequences.

“My feeling towards Hiroshima and the Japanese was, they bloody well deserved it,” he told the Los Angeles Times in 1984. “The whole damn thing has been turned around as if we were the bad guys.”

The blunt-spoken scientist had no qualms about advancing his views in Washington. He advised President Jimmy Carter against a comprehensive ban on nuclear testing in 1978, arguing that it would not only halt the development of new weapons but also weaken the deterrence value of the existing arsenal. The White House ultimately abandoned the idea of a test ban.

At the same time, Dr. Agnew spearheaded efforts to improve nuclear security. In the early 1960s, during an inspection of nuclear caches in Europe, he discovered that the only security on an armed U.S. plane was a lone soldier with a rifle and no training on how to respond to a threat. Under Dr. Agnew’s tutelage, scientists at Los Alamos devised a coded safety system, called the “permissive action link,” to prevent the arming of a nuclear weapon without proper authorization.

Dr. Agnew oversaw the installation of the safety system on all nuclear weapons in Europe as scientific adviser to the supreme allied commander in Europe from 1961 to 1964.

After the Cold War, he was instrumental in molding a new mission for Los Alamos, overseeing the creation of programs unrelated to defense, including projects involving nuclear energy, basic science and biomedical research.

Harold Melvin Agnew was born in Denver on March 28, 1921. He earned a degree in chemistry from the University of Denver in 1942 and joined a research group headed by Fermi, the Italian-born physicist whose work led to the first controlled nuclear chain reaction in Chicago.

He followed Fermi to the Manhattan Project, the top-secret government program at Los Alamos, where the objective was to develop the atomic bomb.

Dr. Agnew was a member of the project from 1943 to 1946. On Aug. 6, 1945, he was aboard a B-29 flying parallel to the Enola Gay when the bomb was unleashed over Hiroshima. With fellow physicists Luis Alvarez and Lawrence Johnson, he measured the yield or force of the explosion using parachutes equipped with blast sensors. The team took turns observing the mushroom cloud through a tiny window. Dr. Agnew had brought along a 16mm movie camera to take footage of the event.

Eighty thousand people died in the explosion and thousands more perished later from radiation and other injuries. An additional 60,000 to 80,000 died as a result of the bombing of Nagasaki three days later.

After the war, Dr. Agnew earned master’s and doctoral degrees from the University of Chicago and returned to Los Alamos in 1949. He continued to work on weapons development while serving as a New Mexico state senator from 1955 to 1961. He was leader of the weapons division at Los Alamos when he was named the director of the 7,000-employee lab in 1970.

In 1979, he left Los Alamos to become president of the San Diego company General Atomics, where he steered the innovation of new reactors, including the helium-cooled reactor. He retired in 1985.

Survivors include two children, four grandchildren and three great-grandchildren.

Although he never had regrets about his role in launching the United States into the nuclear age, Dr. Agnew said he would require every world leader to witness an atomic blast every five years while standing in his underwear “so he feels the heat and understands just what he’s screwing around with,” he told the Los Angeles Times, “because we’re approaching an era where there aren’t any of us left that have ever seen a megaton bomb go off.

“And once you’ve seen one, it’s rather sobering.”

— Los Angeles Times