Pierre Nkurunziza, who became president of Burundi at a hopeful time of reconciliation in 2005, after a bloody civil war, only to become an authoritarian leader whose 15-year reign was marked by repression, violence and an attempted coup, died June 6 at a hospital in the country’s Karusi province. He was reported to be 55.

The government’s official statement described the cause of death as a heart attack, after he was taken ill recently. There was widespread speculation that he may have had covid-19, the disease caused by the novel coronavirus, after his wife was airlifted to Nairobi in May to be treated for the disease.

Mr. Nkurunziza (pronounced en-koo-roon-ZEE-za), who said he believed he was divinely ordained to lead his country, became president at the end of the 12-year Burundian civil war.

The prolonged dispute was largely a conflict between the country’s two primary ethnic groups, the Hutus and Tutsis. The minority Tutsi population had long exercised political and military control in the small central African country, which has a population of about 11 million people and was once under Belgian colonial rule.

Mr. Nkurunziza, whose mother was a Tutsi, identified with his politically prominent father’s group, the Hutus. His father had been killed during a wave of violence in 1972.

Mr. Nkurunziza joined Hutu rebel forces in the 1990s and rose to a position of power in a group called, in shortened form, the Forces for the Defense of Democracy — better known by the abbreviation CNDD-FDD. The group became a political party and Mr. Nkurunziza’s base of power.

By the time the violence ended in Burundi, an estimated 250,000 people had been killed. (The Hutus and Tutsis also battled for control of neighboring Rwanda in the 1990s, leaving about 800,000 people dead in that country.) During the civil war, Mr. Nkurunziza was sentenced to death by the Tutsi-dominated judiciary, but he received a commutation as part of a general amnesty.

As Burundi ratified a new constitution in 2005, Mr. Nkurunziza’s CNDD-FDD party took control of both houses of the national parliament. Those bodies then elected Mr. Nkurunziza president. With his youth and dual ethnic heritage, he held promise as a leader who could heal the bitter divisions in his country.

“The blood shed in the last 12 years is a lot and enough,” Mr. Nkurunziza said when he first took office. “We need to stop the war as soon as possible. We must fight against regional or ethnic divisions, combat poverty through improving health and education.”

During his first term as a president, he brought a measure of stability to Burundi, which is among the poorest countries in the world and was beset by malnutrition and widespread outbreaks of malaria and HIV-AIDS. More than 5,000 schools were built throughout the country, along with 10 sports stadiums.

Mr. Nkurunziza was reelected to a second five-year term, by popular vote, in 2010, but as he tightened his hold on power, he was seen as increasingly erratic and authoritarian.

A onetime soccer coach, he seldom traveled without his personal soccer team, which played local squads around the country. During his years as a rebel guerrilla fighter during the civil war, he became a born-again evangelical Christian after a religious epiphany. He said he was ordained by providence to be president and, in addition to the soccer team, traveled with a choir, which sang at prayer meetings in which the president washed the feet of his followers.

“Nkurunziza indeed believes he is president by divine will,” his spokesman told Agence France-Presse, “and he therefore organizes his life and government around these values.”

Tensions flared throughout Burundi in 2015, when Mr. Nkurunziza decided to seek reelection as president, despite a constitutional limit of two terms. He said his first term — in which he was put in office by legislators, not a vote of the people — did not count.

As violent protests broke out around the country, Mr. Nkurunziza was nearly overthrown in an attempted coup in May 2015. His security forces enforced a brutal crackdown on political dissidents and media that continued through the election, which gave Mr. Nkurunziza a third term as president.

When he was sworn in, he said his enemies would be “scattered like flour thrown into the air — as the God of heaven is a witness.”

In 2015 and 2016, human rights observers from the United Nations counted nearly 500 summary executions in Burundi as well as dozens of disappearances.

Mr. Nkurunziza later ordered the U.N. human rights office to be closed and demanded that international aid organizations report the ethnic identities of Burundians working in their offices.

An estimated 400,000 refugees fled across the border to Tanzania and other countries to escape the rising tide of violence. The brutality was denounced by President Barack Obama and other world leaders, who feared a return of Burundi’s civil war.

In the meantime, Mr. Nkurunziza banned the use of motorcycles in the then-capital city of Bujumbura, suspecting they could be used to spark a revolt. The constitution was amended to allow Mr. Nkurunziza to stay in power until 2034, and the CNDD-FDD pronounced him the country’s “eternal supreme guide.”

In 2019, he announced, much to the surprise of many, that he would not seek reelection the next year. National elections were held last month, in the midst of the coronavirus pandemic, with Mr. Nkurunziza’s handpicked successor, Évariste Ndayishimiye, succeeding him as president.

Pierre Nkurunziza was born in Bujumbura on Dec. 18, 1964. (Some sources report that he was born a year earlier.) He grew up in Burundi’s Ngozi province, and in 1990, he received a bachelor’s degree in sports education from the University of Burundi. He taught and coached at the university and the national military academy.

In 1993, while he was teaching at the university, several hundred students were killed after a military coup in which Burundi’s first Hutu president, Melchior Ndadaye, was assassinated. The experience led Mr. Nkurunziza to join the Hutu resistance. He was wounded several times during the civil war before a cease-fire agreement in 2003.

Mr. Nkurunziza and his wife, Denise Bucumi, had two sons. Other survivors include a sister.

In recent years, Burundi’s military forces have reportedly been trained by Russian mercenaries, and Chinese groups built a multimillion-dollar presidential palace.

When Mr. Nkurunziza stepped down as president last month, he was awarded a luxury villa and more than $500,000 by his country’s government. Burundi’s parliament voted that he permanently be anointed the country’s “supreme leader.”