For more than a decade before he was toppled in a 2017 coup, Mugabe was barred from travel to most Western nations, including the U.S., over claims of torture, electoral fraud and killings of political rivals.

But in the years after he took office in 1980 following the overthrow of white minority rule, Mugabe was a regular visitor to Washington at a time when thousands from the minority Matabele tribe around the southern city of Bulawayo were being killed by a special unit reporting directly to the president.

The targets of the campaign were on the losing end of a post-independence power struggle between Mr. Mugabe and rival guerrilla leader Joshua Nkomo of the Zimbabwe African People’s Union (ZAPU).

The Mugabe government at the time said it was tracking down a small number of ZAPU “dissidents” who had not honored the peace deal and were robbing locals. The former guerrillas also had killed six tourists, including two Americans, near the town of Victoria Falls.

However, reports from churches, non-governmental organizations and journalists showed that, over a period of four years, government forces imposed curfews, burned and bulldozed homes, and herded thousands of civilians into camps, where they were killed and buried in mass graves.

Later research by the Catholic Church revealed high levels of torture and rape. Many victims, it said, were bayoneted to death or burned alive. The church report estimated that 20,000 people died in the action, but a ZAPU party spokesman said it was “at least double that, or more.”

Gregory Stanton, who served in the State Department and is now president of the activist group Genocide Watch, said in an interview that “all diplomatic and intelligence cables relating to the massacres should be released immediately,” along with reasons “why the U.S. took a decision to remain silent.”

In Zimbabwe, the military campaign against the Matabele was known as “Gukurahundi” — a rain that washes away husks after the corn has been reaped.

“This was a genocide, plain and simple,” said Mr. Stanton, who helped set up war crime courts in Rwanda and Cambodia. “It fits the definition used by the International Criminal Court at The Hague.”

With the death of Mugabe, Mr. Stanton said, it is “time for America to clear its conscience.”

Wilf Mbanga, editor of The Zimbabwean newspaper, said London and Washington had taken part in a “conspiracy of silence that included all of Western Europe along with Australia and Canada.”

He said the priority in the 1980s was to end apartheid and white minority rule in South Africa.

“I think many were reticent to criticize someone like Mugabe, who had fought against white rule in Zimbabwe,” he said. “They wanted to show that democracy was working and South Africa had nothing to fear from handing power to the black majority.”

Amnesty International has joined calls for an inquiry into Gukurahundi and said the “failure to hold anyone accountable set a dangerous precedent.”

Critics say the Reagan administration and prominent black Americans at the time repeatedly passed on opportunities to expose and condemn Mugabe’s campaign of violence and terror.

Retired Sen. David Coltart is a former Zimbabwean minister for education. In the 1980s, his Bulawayo law firm represented some of the victims of Gukurahundi.

“It has always been a mystery to me why Ronald Reagan, who was president at the time and who hosted Robert Mugabe at the White House, did not speak out,” he said.

“So many people are still missing, lying in mass graves across Matabeleland. Nothing will bring them back, but answers could help start the healing. Britain and the United States owe them that,” Mr. Coltart said.

In 1983, Newsweek’s correspondent in Johannesburg, Holger Jensen, wrote one of the first stories exposing the Gukurahundi killings. Mr. Jensen later served as foreign editor of The Washington Times.