Yevgeny V. Prigozhin, the outspoken tycoon who built a private paramilitary force that fought on Russia’s behalf in Ukraine and Africa but whose harsh judgment of its army leadership led him to instigate a rebellion, was widely believed to be dead on Thursday, a day after a plane in which he was said to be traveling crashed in Russia. He was 62.

Although his death has not been officially declared by the Russian authorities or confirmed by family members or business associates, President Vladimir V. Putin of Russia spoke of Mr. Prigozhin in the past tense on Thursday and offered condolences to the families of the crash’s 10 victims. And Pentagon officials for the first time openly said they believe that Mr. Prigozhin did not survive the crash, in which all on board were killed. His name was on the passenger list.

Mr. Prigozhin had long leveraged a close relationship with the Kremlin to gain lucrative government construction and catering contracts, and he built up the paramilitary force, known as Wagner, in close cooperation with Russia’s military intelligence services.

For years he kept a low public profile. Even as Wagner conducted operations on Moscow’s behalf in Syria and in several African countries, he denied any affiliation.

Mr. Prigozhin began to embrace a public profile only after Mr. Putin launched a full-scale invasion of Ukraine in February 2022, acknowledging later that year that he had founded and run Wagner. Signing contract soldiers and recruiting prison inmates, he built Wagner into a force estimated at 50,000.

In remarks a day after the plane crash, Mr. Putin said he had known Mr. Prigozhin since the early 1990s — a revelation, since the timing of their relationship had long been a mystery. Mr. Prigozhin once said in an interview that he first met Mr. Putin in 2000.

Speaking in a meeting broadcast on television, Mr. Putin continued: “This was a person with a complicated fate. He made some serious mistakes in life, but he also achieved necessary results.”

He said that Russian investigators would pursue the investigation into the crash “to the end,” and offered his condolences to the families of those who perished in the crash.

Mr. Prigozhin had emerged as a public power player in the past several months, using social media, in particular the Telegram messaging platform, to create a personal brand out of tough talk, obscenity-laced videos and a willingness to endorse extrajudicial killing.

Mr. Prigozhin also became a harsh critic of the way Russia’s military leadership conducted the war in Ukraine, issuing denunciations that Mr. Putin left unchecked even as the government cracked down on other critics. Close enough to the president that he was known as “Putin’s chef,” Mr. Prigozhin had been careful not to take on Mr. Putin directly. And for his part, Mr. Putin seemed interested in creating a sort of competition among his military leaders.

But Mr. Prigozhin escalated his feud with the military leadership exponentially on June 23, saying in a 30-minute video that the invasion of Ukraine was a “racket” perpetrated by a corrupt elite chasing money and glory without concern for Russian lives. Within hours, he had accused the Russian Army of attacking his forces, pledged to retaliate, and deployed his forces on Russian ground itself, soon claiming control of the southern city of Rostov-on-Don, the home of the Russian southern command about 60 miles from the border with Ukraine.

Then, as his troops advanced toward Moscow, Mr. Prigozhin abruptly called off the short-lived mutiny, which had challenged Russia’s veneer of political stability. He agreed to withdraw from Rostov-on-Don under a deal that would drop charges against him and allow him and any fighters loyal to him to leave for Belarus. Since then, he has published videos that appear to place him in locations in Belarus, Russia and Africa, but none could be independently verified.

Mr. Prigozhin (pronounced pree-GOH-zhin) was born in 1961 in the same hometown as Mr. Putin: St. Petersburg, then known as Leningrad. He spent nine years in prison in the 1980s on robbery and other charges. Shortly after he was freed in 1990, just as private enterprise was exploding across Russia, Mr. Prigozhin and his mother, Violetta, set up a network of hot-dog stands. His business interests came to include a supermarket chain, casinos, construction and, eventually, work related to the military.

His company Concord Catering began winning lucrative contracts to supply food to schools, government workers and then the army. According to Russian news media reports, companies affiliated with Mr. Prigozhin persuaded Russia’s Parliament to change bidding laws, a rare step, so they could qualify for military construction contracts worth millions.

Wagner’s roots trace to 2014, when Russia launched its first invasion of eastern Ukraine, a clandestine operation. Mr. Prigozhin sought land from the Defense Ministry to train a private army that could fight on behalf of Russia without an official link to the government. He was given the land, in the western Krasnodar region.

It was a convenient combination, said Andrei Soldatov, an expert on Russia’s security services, because it allowed Moscow to deploy professional, Russian-led military operations abroad while maintaining plausible deniability.

Soldiers trained by Wagner would go on to engage in wars in the Middle East and across Africa, in addition to Ukraine. Human rights groups have accused Wagner soldiers of committing atrocities in Syria, Libya, the Central African Republic, Sudan and Mozambique.

Mr. Prigozhin also oversaw an entity known as the Internet Research Agency, based in his native city, which sought to influence the 2016 U.S. presidential election by spreading false news and disseminating messages supporting Donald J. Trump.

In 2016, the United States imposed financial sanctions on Mr. Prigozhin and several of his businesses in relation to those activities. Two years later, he was one of 13 Russian citizens indicted by a U.S. federal grand jury on charges of meddling in the election. He denied his involvement with the agency for years, just as he had denied his affiliation with Wagner, but after the February 2022 invasion of Ukraine, he went public about his activities.

“I’ve never just been the financier of the Internet Research Agency,” he wrote in a statement to a consortium of Western journalists in February. “I invented it, I created it, I managed it for a long time. It was created to protect the Russian information space from boorish aggressive propaganda of anti-Russian theses from the West.”

Similarly, he acknowledged founding and running Wagner only in September 2022. Mr. Prigozhin’s paramilitary force took on an increasingly public role after the Russian Army’s plan to capture Kyiv immediately in the initial days of the war had failed. He was seen in videos personally recruiting inmates from prison to serve in Wagner.

“If you serve six months, you are free,” Mr. Prigozhin told a group of uniformed inmates in one video published in September. But he was also blunt about their fate should they prove disloyal. “If you arrive in Ukraine and decide it’s not for you, we will execute you,” he said.

He was able to recruit many thousands of prisoners before announcing in February that he would stop the practice.

As his soldiers were thrown into brutal battles for control of cities in eastern Ukraine, including Bakhmut, Mr. Prigozhin regularly recorded videos near the front lines, seeking credit for battlefield victories, praising his troops as “probably the most experienced army in the world today,” and lambasting the Russian military leadership. In domestic polls, he became one of the best-recognized political figures in Russia.

On his social media channels, and on a media outlet he controlled, RIA FAN, he regularly published videos that struck populist themes and drew implicit contrasts with the Russian defense minister, Sergei K. Shoigu, who had never served as a soldier. Mr. Prigozhin frequently traveled to Wagner encampments, and his criticism of the military leadership and Moscow’s “elite” became blistering as the war continued.

He also embraced ruthless violence against perceived enemies. When a video surfaced of a man who had defected after agreeing to fight for Wagner being killed with a sledgehammer, Mr. Prigozhin appeared to endorse the action.

“A dog’s death for a dog,” he wrote.

Recently, he called for the imposition of martial law and the cessation of normal government activities in Russia, claiming such steps were needed to win the war.

“We must stop building new roads and infrastructure facilities and work only for the war, to live for a few years in the image of North Korea,” he said in an interview published on the Telegram messaging platform in May. “If we win, we can build anything.”

Mr. Prigozhin was often seen as a creation of Mr. Putin, who analysts say watched and even enabled his public feud with Mr. Shoigu to spur competition for results. But many wondered how long it could continue before escalating into something violent.

In the Telegram interview, Mr. Prigozhin forecast political violence caused by relatives of dead and wounded soldiers increasingly disillusioned by what he described as a coddled elite.

“Society always demands justice,” he said, “and if there is no justice, then revolutionary sentiments arise.”

Oleg Matsnev contributed reporting from Berlin, and Anton Troianovski from London.