William B. Bader with his wife, Gretta. (Courtesy of the Bader family/Courtesy of the Bader family)

William B. Bader, who held high-ranking foreign-policy positions with several federal agencies and who, as a Senate staff member, helped investigate CIA abuses and events surrounding the 1964 Gulf of Tonkin Resolution, died March 15 at a care facility in Sykesville, Md. He was 84.

He had complications from Alzheimer’s disease, said a son, Christopher Bader.

While working for Sen. J. William Fulbright (D-Ark.) in the late 1960s, Dr. Bader was among the first people to cast doubt on the official reasons given by the Defense Department and the White House for escalating U.S. military involvement in Vietnam.

On Aug. 4, 1964, President Lyndon B. Johnson went on national television to announce that the U.S. military was taking action against “repeated acts of violence” by North Vietnamese forces. According to the Defense Department, Navy ships had come under fire on two occasions in the Gulf of Tonkin off the coast of Vietnam.

The first attack, on Aug. 2, was on the destroyer USS Maddox. Two days later, defense officials said the Maddox and a second destroyer, the USS Turner Joy, had come under automatic weapons fire and torpedo attacks. The Maddox fired hundreds of shells during the nighttime incident, and U.S. jets were dispatched from a nearby aircraft carrier.

Johnson used the episodes as justification for the Gulf of Tonkin Resolution, which was passed by Congress on Aug. 7, 1964. The resolution authorized the president to “take all necessary measures” to protect U.S. interests and led to a decade-long military engagement in Vietnam that claimed about 58,000 American lives.

Dr. Bader, a onetime naval intelligence officer who worked at the CIA and State Department early in his career, was a member of Fulbright’s staff in 1967, when he began to examine Navy documents concerning the Gulf of Tonkin incidents.

There was no doubt that the Maddox had exchanged fire with a North Vietnamese vessel on Aug. 2, 1964. But Dr. Bader helped raise questions about the second attack, finding no evidence that it had taken place.

Fulbright, an early critic of the Vietnam War, charged then-Defense Secretary Robert S. McNamara with misrepresenting evidence about the supposed assaults. Fulbright suggested that the Gulf of Tonkin Resolution had been passed under false pretenses.

Many documents related to the episode were not declassifed until 2005 and 2006, when the doubts voiced by Fulbright and Dr. Bader almost 40 years earlier were confirmed.

A 2008 article in Naval History magazine, written by Navy officer Pat Paterson, concluded that “high government officials distorted facts and deceived the American public” about the Gulf of Tonkin incidents.

One of the Navy pilots sent out from the USS Ticonderoga to attack North Vietnamese vessels was James B. Stockdale, who later became a vice admiral and the 1992 running mate of independent presidential candidate H. Ross Perot.

In his 1984 autobiography, Stockdale was clear about what he saw: “I had the best seat in the house to watch that event and our destroyers were just shooting at phantom targets . . . there was nothing there but black water and American firepower.”

William Banks Bader was born Sept. 8, 1931, in Atlantic City, where his grandfather had been mayor in the 1920s.

After Dr. Bader’s father was killed in an automobile accident in 1934, the family moved to Los Angeles. Dr. Bader graduated from Pomona College in Claremont, Calif., in 1953. He then studied in Europe on a Fulbright fellowship — an international academic program sponsored by the senator he would later work for.

He served in the Navy from 1955 to 1958, received a doctorate in history from Princeton University in 1964, then worked for the CIA and the State Department for a few years. He published a book, “Austria Between East and West,” in 1966.

In the mid-1970s, Dr. Bader was on the staff of a Senate investigative committee led by Frank Church (D-Idaho). In that role, Dr. Bader helped expose a variety of unsavory practices by the CIA, including attempts to topple governments and assassinate foreign leaders.

Dr. Bader later worked at the Defense Department before returning to the Senate as chief of staff of the Foreign Relations Committee from 1979 to 1981. He then spent 10 years with SRI International, a research firm and government contractor. He was president of the Eurasia Foundation in Washington from 1992 to 1995 and, over the years, lectured at many universities.

He was an assistant secretary of state for educational and cultural affairs from 1999 to 2001.

Dr. Bader lived for many years in Alexandria, Va., and was a member of the Cosmos Club and Western Presbyterian Church in the District.

His wife of 60 years, sculptor Gretta Lange Bader, died in 2014. Survivors include four children, Christopher Bader of Medford, Mass., Katharine Bader of Durham, N.C., John Bader of Kensington, Md., and Diedrich Bader of West Hollywood, Calif., an actor in the cast of the HBO series “Veep”; a brother; and six grandchildren.

In 1998, Dr. Bader spoke at a ceremony in Fayetteville, Ark., honoring Fulbright, his onetime mentor. He recalled how the Fulbright fellowship had helped shape his life and those of countless other young people.

“None of us had ever been out of the country,” he said. “We learned, we saw and we were changed.”