A preliminary UU intelligence assessment concluded that an intentional explosion caused the plane crash that killed Wagner Group leader Yevgeny Prigozhin, who was eulogized Thursday by Russian President Vladimir Putin and had posed the biggest challenge to the Russian leader’s 23-year rule, AP reported.

One of the US and Western officials who described the initial assessment said it determined that Yevgeny Prigozhin was “very likely” targeted. 

The officials, who spoke on the condition of anonymity because they were not authorized to comment, did not offer any details about what caused the explosion.

Several of Prigozhin’s lieutenants also died in the crash.