Sergei Ivanov, a former Russian defense minister and a close ally of President Vladimir Putin who was once considered a potential successor, has died at the age of 73, the Kremlin announced on Friday. No cause of death or further details were provided. President Putin extended his condolences to Ivanov’s family.

Ivanov and Putin’s relationship dates back to the 1970s when they served together as KGB officers in Leningrad, now known as St. Petersburg. Their longstanding connection laid the foundation for Ivanov’s key roles within the Russian government over subsequent decades.

After Putin’s initial election as president in 2000, Ivanov was appointed secretary of Russia’s Security Council. In 2001, he became defense minister, a position he held until 2007. During his tenure, Ivanov oversaw Russia’s military efforts in the second Chechen war, which resulted in the suppression of separatist forces in the region.

Following Putin’s move to the prime ministership in 2008 due to presidential term limits, Ivanov was widely regarded as the leading candidate to succeed him as president. However, Putin chose Dmitry Medvedev, another longtime associate, as a transitional figure until Putin returned to the presidency in 2012.

Some analysts believe Putin rejected Ivanov’s potential candidacy out of concern that Ivanov’s ambition might threaten Putin’s control if Ivanov sought to maintain the presidency beyond a transitional period.

Despite this, Ivanov remained a prominent figure in the government, serving as deputy prime minister from 2007 to 2011, and subsequently as Kremlin chief of staff from 2011 until 2016. His death marks the passing of a significant figure in Putin’s inner circle during a critical period of Russian politics.