When Burundi became the first country to withdraw from the International Criminal Court less than two weeks ago, its leaders thought they had successfully avoided an inquiry into egregious crimes including murder, torture and rape.
They apparently miscalculated.
The court announced Thursday that a pretrial panel of judges had authorized the prosecutor, Fatou Bensouda, to open an investigation concerning crimes committed while Burundi was a member.
The judges also said that Ms. Bensouda had the authority to investigate any related crimes committed since Burundi’s withdrawal, “if certain legal requirements are met.”
Burundi’s legal obligation to cooperate, the judges said, “remains in effect for as long as the investigation lasts and encompasses any proceedings resulting from the investigation.”
Their announcement said the judges had given approval for the inquiry on Oct. 25 — two days before Burundi’s withdrawal from the court took effect — but that the decision had remained under court seal until Thursday to protect victims and potential witnesses.
The decision stunned and infuriated the government of Burundi, the tiny southern African nation that is the first — and so far, only — country to withdraw as a party to the Rome Statute, the treaty that created the court in 2002.
Willy Nyamitwe, the senior adviser and spokesman for President Pierre Nkurunziza of Burundi, said in a Twitter posting that the court’s decision reflected what he called its “outrageous lies to implement Westerners’ hidden agenda to destabilize #Africa.”
Burundi’s withdrawal has come to symbolize resentment by African leaders toward the Hague-based court, which has largely focused on crimes committed in Africa. Still, other African nations have not followed Burundi’s lead, despite initial predictions they might.
The Burundi investigation is likely to focus on evidence of extrajudicial killings, disappearances, arbitrary arrests and detentions, torture and sexual violence in the two-and-a-half years since Mr. Nkurunziza forced and intimidated his way into a third term in office.
In September, a United Nations human rights panel found that Burundi’s top leaders and security agencies were implicated in such crimes, and it urged the International Criminal Court to open an inquiry.
Rights advocates welcomed the announcement by the court on Thursday. Param-Preet Singh, associate international justice director at Human Rights Watch, said Mr. Nkurunziza’s loyalists in the country’s security forces and the police had a “devastating track record of unchecked abuses” that had invited scrutiny by the court.
“I.C.C. involvement means victims in Burundi and their families may one day see those responsible brought to justice,” she said.
Some lawyers and human rights activists have said that the court’s prosecutors were obliged to make good on their planned investigation of Burundi, and needed to send a signal that leaving the court was not a guarantee of immunity.
But court officials concede that experience has shown that leaders in power can effectively block investigations. Whether the investigators can do their work if the government thwarts them remains an open question.
According to the Rome Statute, if a state withdraws from the court, it is still obliged to cooperate with a criminal investigation or proceeding if it was underway before the date of its withdrawal.
In Kenya, where President Uhuru Kenyatta and Vice President William Ruto were elected despite facing charges before the court, proceedings against them collapsed because of what prosecutors and judges called witness interference, including bribes and threats. Court investigators also say they were blocked from talking to police and other government employees, and denied access to banking and telephone records.
In Sudan, the prosecutions of President Omar al-Bashir and some of his close aides were suspended after investigators failed to gain access to witnesses and victims within the country, and after the court failed to secure the arrests of the accused even when they traveled.
It was Mr. Bashir who began a campaign in the African Union to undermine the court, which was joined several years later by other presidents: Mr. Kenyatta of Kenya, Jacob Zuma of South Africa, Yahya Jammeh of Gambia.
Their efforts to organize a mass walkout from the I.C.C. have so far failed.
Kenya has so far not acted on its plans to leave. Gambia reversed course after Mr. Jammeh was voted out of office. South Africa remains a member because its high court ruled that the nation’s Parliament, not Mr. Zuma, had the last word.
Human rights groups and lawyers have pushed back with their own campaigns to sway African opinion, arguing that those who oppose the court are only trying to protect themselves.