Wagner Group mercenaries came dangerously close to entering a Russian nuclear base to steal nuclear weapons that could fit into a backpack to use as leverage during their short-lived mutiny last month, according to Ukraine’s chief spymaster.

While the main force of the rebels, directed by Wagner Group founder Yevgeny Prigozhin, rumbled toward Moscow on June 24, purportedly to capture Russia’s top military brass, a smaller group drove east en route to a fortified army base.

Kyrylo Budanov, the 37-year-old head of the Ukrainian Defense Intelligence Directorate, said the mercenaries’ target was Voronezh-45 — a well-guarded facility that purportedly stores small, Soviet-era nuclear bombs that can be carried by a single person in a backpack.

“Because if you are prepared to fight until the last man standing, this is one of the facilities that significantly raises the stakes,” Budanov told Reuters in an exclusive interview.

The mutineers’ plot to get their hands on nuclear weapons only failed, according to Budanov, because “the doors of the storage were closed and they didn’t get into the technical section.”

The Ukrainian official did not say why Prigozhin’s fighters left the nuclear base without trying to force their way inside.

A source close to the Kremlin with ties to Russia’s military confirmed parts of Budanov’s account, saying that Wagner fighters “managed to get into a zone of special interest, as a result of which the Americans got agitated because nuclear munitions are stored there.”

The rebels’ attempt to steal nuclear explosives was apparently so alarming to the Kremlin that it prompted President Vladimir Putin to agree to a peace deal with Prigozhin, which was hastily brokered by Belarusian leader Alexander Lukashenko, according to a source in Russian-occupied eastern Ukraine.

Under the agreement, the Wagner rebels avoided prosecution for treason and were allowed to either follow their leader Prigozhin into exile in Belarus, or join Russia’s regular army.

Prigozhin’s fate remains unclear. According to Kremlin spokesman Dmitry Peskov, Prigozhin returned to Russia on June 29 to meet with Putin to discuss the mutiny, and Lukashenko said last week that he was not in Belarus. The Wagner boss was also recently sighted in St. Petersburg, arriving at the local FSB office to collect his arsenal of weapons that had been confiscated in June.

US officials questioned Budanov’s unconfirmed claims concerning Wagner fighters’ side trip to the Voronezh-45 base, insisting that Russia’s nuclear arsenal was never in danger during the brief insurrection.

“We are not able to corroborate this report. We had no indication at any point that nuclear weapons or materials were at risk,” said White House National Security Council spokesman Adam Hodge.