Ukrainian authorities Monday placed former president Petro Poroshenko under formal investigation for high treason. Poroshenko is accused of facilitating the financing of separatist forces in eastern Ukraine.

Poroshenko served as Ukraine’s fifth president from 2014 to 2019. During that time, a conflict between Russian-backed separatist forces and Ukraine broke out in the Donbas region in eastern Ukraine. Ukraine’s primary coal mines were located in the Donbas region. When the region was seized by separatist forces, Ukraine faced an energy crisis from coal shortages.

The Ukraine State Bureau of Investigation (SBI) unveiled the formal investigation of Poroshenko in a statement Monday. SBI accused Poroshenko of facilitating the purchase of coal from separatist forces from 2014 through 2015. SBI asserted Poroshenko was involved in a coal purchasing scheme which provided 1.5 billion hryvnia (approximately $55 million) to separatist forces.

The separatist forces referred to SBI’s statement are the Donetsk People’s Republic (DPR) and Luhansk People’s Republic (LPR). SBI described both groups as terrorist organizations. SBI argued that coal purchases made in 2014 through 2015 caused Ukraine to become dependent upon LPR, DPR and Russia for energy. As a result, Russia was given a greater opportunity to interfere in Ukraine’s internal affairs.

With the announcement of SBI’s formal investigation, Poroshenko becomes the fourth suspect involved in the coal purchasing scheme. Leader of pro-Russian party Opposition Platform Viktor Medvedchuk, Ukraine ex-minister of energy Vladimir Demchishin and businessman Sergei Kuzyara are also under investigation.

Poroshenko faces up to 15 years in prison if he is found guilty.